1997 Buick lesabra

We had a mechanic put a plenum on the top of the engine because water was starting to get in the head of the engine, but not in the oil. He told us the plenum was bad and needed to be replaced. He changed the plenum, changed the oil, flushed out the radiator, replaced the fuel filter. There is no water build up now, and no water at all in the oil. We drive and it drives well for about 15 minutes and then while you drive, it dies like it doesn't has no gas. Waiting about 10-15 minutes after it dies the first time, it will start back up and run about another mile then dies again.