Car Stalls While Driving or Traffic Stops Occasionally

I drive a 1998 Buick LeSabre Custom V-6 3800 Series engine. Mileage is 92,000. For six months it occasionally will die out while driving when engine is warm and sometimes will die at stop signs. I have changed the coolant temp sending unit, the crankshaft positioning sensor, fuel filter is one yr old. Normally runs great and idles fine. No engine light comes on or codes have ever came up.
Always restarts right back up. Seems to happen more in very hot weather (90's), but has happened rarely in lower temps. Has happened as few as 2 times and as many as 9 times in one day but not after dark. Has always been serviced faithfully by dealer, but the mechanics are now stumped. HELP is desperately needed ASAP.