93 Bonnieville stalling

I have a 93 Bonniville which we changed the transmission and motor from another 93 Bonniville (I believe identical motor & tranny). Since doing this have starting having a problem with stalling. The symptoms are when we get into the cold car usually within running for anywheres from 5-20 minutes the engine will stall as if you just pulled the plug on it. No sputtering or hesitation - just stall outright. It sometimes will start back up immediately but more often you have to either sit and wait anywhere from 3 to 5 minutes. The engine is turning over fine and have fuel pressure on fuel rails - just ignition will not kick in. We have also on occasion moved the car by pushing a short distance (10-20 foot) and gotten back in and it started.

This stalling usually occurs when we are coasting to a stop or a turn or release the throttle downhill. It has always restarted with a wait time or a movement of the vehicle by pushing or pulling.

ANY HELP OR SUGGESTIONS WOULD BE GREATLY APPRECIATED.

I forgot to mention in my response to your PM, check the crank position sensor. It could be causing the entire problem. They will act up intermittantly and cause the car to stall. Check this first!