My 98 GT's been acting wierd, Engine starts and stops Options

[RavenAegis45]

everytime I start my car it'll start but the RPM's will quickly drop and it'll turn off, however if I rev my engine usually around 2.5 to 3 RPM's it'll stay on but i notice a puff of White Smoke from the exhuast and it only puffs after i Rev? Any ideas what it might be?

[ferngt4]

my 94 st had somewhat the same problem. RPM would just drop and car would die but would start back up if car was moving. Never figured out what it was, even after hooking up every scanner/scope available.