This happened three times during my ownership of the car as follows:
(1) 2016: I was on a highway at the speed of 100kph in a cold winter night then suddenly the car started to shake. The shaking feel first came from the steering wheel then spread to the whole car body. **The shaking did not go away even when I slowed down to 30kph**. I drove off the highway to check and didn't see anything obvious. Then I got on the highway again and the problem went away. Since this was the first time, I intuitively thought it could be related to the drive shaft. A few days later, the drive shaft was replaced. I thought I made a wise judgment and took a conscious action since the problem didn't come up again until...
(2) 2018: I was in a highway tunnel at the speed of 90kph then suddenly the car started to shake. I was lucky to be able to find an emergency stop ahead. The car was later towed to the shop. The thorough checkup did not reveal anything wrong (balance, alignment, joints, shafts, ...). So I continued to drive the car and went back to the highway and the problem just went away. Although exactly the same shaking feels like the first incident, I didn't consciously link this incident to the first one since the shafts were replaced after the first incident.
(3) 2021/10: I was on a highway at the speed of 100kph at night then suddenly the car started to shake. This just happened last week and I consciously linked this one to the previous two this time. So I drove off the highway and came to a full stop then turned off the engine with previous experience (the reason to turn off the engine is my suspect of engine or stability control malfunction in the hope of resetting it other than the mechanical balance issue). And, as expected, when I re-started the car and went back to the highway, everything was back to normal and the problem went away.

My question is: If this is coming from mechanical imbalance, why is that every time the problem seemed going away after a full stop? But there's an obvious common in all these three incidents which are they all first happened at highway speed. Also, maybe a joke, they all happened in dark ambient light (night or in the tunnel).

 

I would assume you've been through multiple sets of tires during this timeline, right? If so, I would rule out tires. Perhaps a couple of your wheels are out of true and when their imperfections line up, at speed, it causes a harmonic imbalance. Have you rotated your wheels/tires often during the timeline? Any damage to your wheels?

 

Actually, in the latest incident, the tires were just replaced and trued a month ago. What you said could very well be the cause of it. But still, what puzzles me is that in all three incidents, the shaking, although decreased in degree as I slowed down, did not completely go away until I made a full stop.

 

Could be tires out of balance? You need to start eliminating things to get to the bottom of it. Try road-force balance first on front tires. I’ve had tire places doing horrible job balancing new tires, so you need to rule it out first

 

My guess is that the drive belt or something along those lines may be failing. Since the steering wheel seems to shake when this is happening, it may be that the belt slipping or binding causing the power steering pump to engage and re-engage. Especially at high speeds on the highway, this could cause the entire vehicle to shake.

 

Thanks for the pointer. Road-force balance is new to me. Definitely, something to check out at the mechanic place.

 

These are learned suggestions. In fact, I did notice the belt making a screech noise occasionally these days. Definitely something that can be easily taken care of. Thanks!

 

I'm not a fan of "road-force" balancing as it can correct for defective tire or wheel, masking the underlying problem. It's better to determine which wheel or tire is causing the issue and replace it. Otherwise, you're kicking the can down the road.

 

Thanks for the answer which gave us a hint in the process of solving the problem.

The same problem happened twice again last week on different days both after two hours on the highway and around 65mph. In fact it was nearly at the same spot of the same highway and 2 hours from where I live but I don't think the location matters . On the 2nd incident of last week, I noticed that I have to stop the car, shut down the engine, and wait about 10 min to cool down a bit. Otherwise the wobbling (I wouldn't use the word "shaking" since it feels like your steering wheel was guided by the wheel when the car started moving from still). I managed to drive home at a much lower speed (40mph) since it appeared to be a bit consistent that speed above 65mph could trigger the problem (again just a reminder here: The problem, once happens, will continue until the car is fully stopped).

So, I went to the shop that didn't find anything wrong last time the next day after the 2nd incident of last week. This time we all were more serious about the problem. The mechanic requested to test drive with me and we did but I told him upfront that you'll have to drive for at least two hours at highway speed. A bit suspicious on his face, of course, the problem didn't surface during a 20-minute short test drive although at highway speed. The two hours of driving "requirement" did lead us to suspect that somewhere in the problem chain was causing high temperature which ultimately triggers the problem. But "high temperature" in where? The mechanic checked and found no leaking or obvious sign of broken steering parts around and suspected that it could be caused by dirty power steering fluid and BINGO. Even though I insist on replacing the whole steering components (power steering, pump, and of course the fluid), the mechanic suggested flushing and replacing the fluid first since the cost of replacing the whole thing wasn't cheap. So, we did just the fluid. And voila, I've since done a few more trips (4 hours of highway driving) on the same route where it happened before. And the problem was gone. I was surprised that the dirty power steering fluid can have such a big impact on the problem. And of course, saved big $$$ on this.

 

Great find and it is easy to change the power steering fluid yourself.
Get a Turkey Baster, suck as much old fluid as you can from the reservoir and fill with fresh power steering fluid (a/k/a Dexron III ATF).
Drive around as much as you want and repeat a few more times.
The total volume of fluid is around a quart which is why doing this 3 or 4 times will replace almost all the fluid.

 

I've never heard of dirty PS causing a vehicle to shake. Heat was not the trigger because the shaking didn't occur every time the car warmed up. I'm thinking more along a bad motor mount or harmonic balancer. Check the upper 'dog bone' mount. The front mount has a liquid chamber as well as a vacuum diaphragm that are used to cancel out vibrations. I will be very surprised if the vibrations never return. For your sake, I hope they don't.