Pontiac Repair: montana 1999 stalls, Montana Stall issue

Question
my 1999 montana has always run great except at speeds over 50mph.  Mechanic put it on the computer and said the mass air flow sensor needed replacement.  My BF thought he could do it cheaper so he replaced it and the air filter last week.  Now it runs but starts to chug, and then stalls and wont stay started.  I got it home, but what could this be?  BF is up north, maybe that's a good thing....Thanks...ps: took the gas cap off and it ran ok, could be a coincidence

Answer
Hi Marsha:

Taking the gas cap off could be a fluke more than likely! when it stalls does it chug all the time then stall or is it like someone turning the key off? If it spits and chugs before it dies I'd look at a fuel pressure issue. Either a pump that's going bad or even a plugged fuel filter. That is what I would look at first. Very rarely does a Mass air meter cause stalling on these engines they usually cause hesitation and service engine lamps to come on more than anything else. Good luck :0