2004 Chevy Malibu 3.5 V6 engine start problem

I am having the same problem with 2004 Chevy Malibu 3.5 V6. Sometimes the car won't start - the engine turns over, so it's not a dead battery, but the engine won't start sometimes. When I have it towed to dealer, it cranks right up. Something to do with the motion of being on the flatbed tow truck. So now when it won't start, I try pushing up and down on the front end, and that usually works - it cranks right up. Dealer can't really figure anything out. They replaced the fuel pump relay and cleaned the throttle body, but I'm afraid the problem might happen again. Did you ever figure out definitely what the problem was with your car? Tony.james@cna.com