1991 Buick Century/ car shuts down after 15-20min

1991 Buick Century, 125,000 miles, 3.3 engine Problem happened when I filled up with full tank of gas. Stayed stranded at the gas pump. The car cranked and acted like it wanted to start and didn't. After 15-20 minutes car started, made it home. I got home and car stranded again. I tried restarting car and same thing happened, it just cranked but car would not start. Once car cools down about 15-20 minutes, car turns on again. This is continuos, car runs well for the first 15 minutes then shuts off. So We checked coil packs, they are good, we replaced Ignition control module problem was not fixed. We took the alternator off and had it checked at 3 different locations, told alternator was bad. We put in a new alternator, still did not fix the problem. Replaced spark plugs and still did not fix the problem. Please Help, what do I check next?