Here is a stumper. We just had a new (used) engine put in the car. Only in the morning (and it is humid and muggy) does this happen. Car will start and will run well. Then, while driving - it will stall or at a stop light - will stall. It will start right back up. It hasn't happened in the afternoon - only in the morning after the car has been sitting over night. Any ideas? Engine related (so it is under warranty)? Our mechanic is swamped and is trying to fit us in to look at it - but would like some thoughts if anyone has one!