1990 Cadillac Seville, 150,000 miles, 4.5 L engine. Most of the time this car will not start again after you have driven a while and the engine is warm. If you try to restart it within a few minutes, it will not start. If you wait anywhere from 1-4 hours it will. It has been in the shop twice now, they of course cannot get it to fail to see what the problem is. Any ideas??