Car starts then dies....Help please!!

Ok, so over the last few months every once in a while my car will start up then immediately die. I crank it up again, and its fine. On Thursday, it kept doing this and I could not get it to stay running after numerous starts. It died on me everytime I took my foot off the gas. I tried to use the transmission and downshift to help stop me while driving, and when I did the car would backfire badly. I brought it in Friday morning to a mechanic and he said my fuel pump and fuel filter were bad. So $750 dollars later I get the car back and it seems to run fine. The following day, it did the same thing, but on the second start it worked fine. Today, once again couldn't get it to run. The mechanic said the "bad" pump was only putting out 25-30 lbs of pressure. He said there were no codes found in the computer (but I do have MIL eliminators for the off-road H-pipe....I think this shouldn't matter for other problems though).

Any suggestions? Did I take it to an idiot mechanic?

I can try that, but I would think with a brand new fuel pump and filter I shouldn't have to. I am pretty sure the problem I have is not the one that was fixed and wondering what that problem would be. Usually once I get it started and it doesn't die on me immediately, it will seem to run fine.