1991 Porsche 944. starts fine, runs, then stalls...repeats..

I have a 1991 Porsche 944. I just had an old change a few weeks ago. It always starts fine, but when I stopped for gas it wouldn't start. I tired over 30 minutes and finally towed it home. It started fine several times the next day. When I have driven it since, one time it seemed to loose power and I down shifted, and pulled over to the side of the road and itthe power came back just fine. Tonight when I drove it, it powered down again and will turn over, sometimes start and then just dies. AFter 15 minutes it started and I dove off again only to have the same thing happen again. I waited 30 minutes, started again and then 10 minutes down the road, it dies again, will actully turn over and start then dies again. What could this be?