99 cavalier acts like out of gas when cold, eg. outside temperature under 40 degrees F

I don't usually suggest parts replacement without testing first - however i will make an exception here : Replace the engine *coolant temperature sensor*..

My wife's 99 Cavalier has his exact same problem. Tried replacing fuel pump, fuel filter, fuel regulator, battery, throttle position sensor - nothing. One of her co-workers suggested placing an electric heater in the cab ten minutes before starting (apparently she had a cavalier with same problem). As you can imagine, my response was "that's not gonna work. Sounds dumb". However, out of frustration, decided to give it a shot (after all, what can it hurt. Right?). IT WORKED LIKE A CHARM. Problem still exists, and I still don't know the cause, but in the winter, this is what we have to do every morning. Summer is fine. No need for the heater. And car runs great. In fact it's one of the most reliable cars we have ever owned. But when winter rolls around, out comes the heater and extension cord. Runs fine once it starts, and stays running for the rest of the day, starts right up. But initial start requires the electric heater. Any ideas? I'm still stumped. And in awe that her co-workers idea actually worked. Mike V. Albuquerque, NM