2002 Chevy Tahoe wont run

I had to take the tahoe in to get a smog cirt. And all though it passed emissions the check engine light was on so the mechanic had to replace an oxygen sensor & the t.P.S. I was told to put a few gallons of fuel in it because of an evapuration test the on board computer had to run, that being done I took it back the next day & it passed. The following day after about putting 6-8 miles on it, it died. It would barely run and dieseled bad, I had to have it towed home. I called the mechanic and he told me it sounded like a bad fuel pump, I disconected the fuel filter & turned the key and the fuel pump squirts out fuel. When the line is reconnected the engine starts but runs rough, once the engine is turned off it will not restart. I have replaced the filter with the same result. My mechanic denies any responsibility but says to bring it down allthough it will have to be towed. My question is can a fuel pump run weak all of a sudden or does it go all at once? Any suggestions? Thanks Fred