1996 Cadillac Seville starting

At times this car will not start. A shot of starter fluid and it cranks and runs fine. This does not happen all the time, just frquently enough to be a bother. The fuel presure regulator has been replaced, the 2 second pump-up is fine, fuel pressure us 55 lbs, even atstart-up. Car runs great once it starts. Seems this problem occures more frequently after the car has been idle for several days. Also, seems to be more of a problem since temperture change in the fall. We live in metro Atlanta where they use fomulated fuel in the summer for polution and I do not know if this might be a contributng factor. Have taken it to several mechanics but it starts for them so they cannot replicate the condidion to diagnose. Any suggestiond would be appreciated.