Kingosme

Okay, lets start of with my car has about 144k miles, and i absolutely love this car; and so my issue has been very weird. So i replaced the radiator with a new radiator, the old one had a crack which looks like it happens quite often. Since i replaced the radiator (i flushed the coolant of course) the car has been absolutely flawless for about 2 months, and so what has been happening recently is when my car is in idle the fans kick on and the temperature needle is steady in the middle. But when i shift into drive and drive around local roads at low rpms the needle starts to rise. and when i go onto the highway the needle goes back down to the middle. I am so confused as to where to even start. Any suggestions?