my 99 gs 400's engine shut off automatically while i was driving in the parking lot. this happened twice with in the past 6 months. and this happens at very low speed (5-10mph). i am scared to drive my baby now until a lexus service depatment checks it out. i am driving my accord for now. ever heard of any similar problems? PLEASE HELP.

 

Would this happen to be RIGHT AFTER you've bought gasoline, like while driving out of the parking lot of the gas station at low speed?
This happened to me three times (all at about 5-10 MPH), until I realized that the car ('99 GS400) just cannot tolerate being gassed up while the ignition is on. If the gas tank was less than 1/4 full prior to refilling, it apparently allowed air to get into the fuel lines (from the bubbling of the refill), and would shut the engine down. Hasn't done it to me again since I realized this. Pretty damn scary, though. I've gotten rid of cars for less unpredictable stuff than this.
If this is not your case, it may still be fuel-pump related, or it could be engine control module. Get it checked out at the dealer, but beware - Murphy's Law says that these problems never re-occur while at the dealer, so since it is intermittent they may not be able to find the cause. You might want to change your brand of fuel and the gas station that you normally go to as well, just to eliminate that as a possibility.
Good luck.