I start my car fine, but then when I put it in drive or reverse, the car dies. This happens even when the car has been sufficiently warmed up. The other day, after dirivng about 160 miles round trip, I started the car and the engine died every time after 10 attempts and after revving the engine for several minutes straight. The mechanic thinks it might be due to a valve but when I brought the car to him the next day, it was fine and he could not replicate the problem. What could be causing this, I can't keep driving the car around not knowing when the next time it's going to die and not start up.