1985 Buick Riviera keeps stalling out!

ok, this is a bit long winded but i want all the information there as to get a proper assessment.
i bought my buick about 10 days ago and have been having nothing but problems with it. the first time
it stalled out was shortly after i added new gas to it so we though it could be old/bad gas mixing
around with new gas. added some fuel treatment and replaced the fuel filter. would still stall if i was
stopped in drive but would continue to run if i threw it into neutral at stop lights. decided to give it a
tune up so i replaced all the bad vacuum hoses, spark plugs, plug wires. got it an oil change, filled the
all the fluids up and burned all the gas i had added out and added fresh gas with fuel treatments. ran
like a dream. for a day. yesterday i took it on a drive and had to grab some gas on the way into town.
10 minutes later i parked downtown, had dinner, then when i was leaving was having a really hard time
getting it going. it was dying in park which it wasn't doing before. before i could at least count on it to
run in park or neutral just not in gear (while stopped). got it going and it died while driving (i think my
foot was off the gas). finally got it home. started it this morning and i can't even get that sucker
running long enough to get it out of my driveway. what do i do? is this a case of bad gasoline or
something else? i'm at a loss. i don't have a lot of money so i'm trying to figure it out myself. all
suggestions welcome.
thanks,
kiley