1998 Pontiac Grand Am Engine keep stalling in real hot weat

Problem begins after the car has warmed up completely. Engine runs fairly well at high speed, stalls in stop and start traffic or when having to slow down but not stop. Engine usually restarts after it stalls, but, after stalling numerous times, does not readily restart. Sometimes will cut-out while running, or significantly lose power. This happens only when outside temperatures are over 85-90 degrees. This does not always happen in hot weather, and has never happened in cooler temperatures. This has been a problem since the car was new, but recently the problem has gotten much worse. Because it happened so infrequently in the past, no one has ever been able to diagnose the problem.