1998 lumina just stops

I just bought this 98 lumina and it ran great for 2wks. Then all of the sudden it will just slow down while I'm driving. I step on the gas and nothing happens. I then have to pull over and restart the car after a minute it will do it again. I tried gumout and filled up the tank. It ran for 40 minutes after that and then stopped again. I took it to chevy and they have no clue maybe the fuel pump. I took it to another shop the suspect the fuel pump or maybe egr valve or a map sensor. Oh and their are no codes in the computer according to chevy. Any ideas anyone?

Someone is going to have to at least check the fuel pressure when this happens, or see if it is up to specs under load, basic diagnostics here for step one.