2002 Pontiac Grand Prix 3.1 V6 very rough idle

My husband and I just got a 2002 Pontiac Grand Prix.
(Roughly 225000 miles on it)  It worked absolutely
perfectly when test driving it and nothing looked
obviously wrong, so we got it as we are in desperate
need of a vehicle. Worked fine for a few days, then
when driving down the highway it just shut off. Earlier
that day we had noticed a slight gurgling sound in the
front end, passenger side. Anyway, I pulled over,
turned the key and shut it all the way off, then turned it
back on. Great, no hesitation, ran fine. Get it back to
our house, don't use it til the next day, and when I tried
to turn it over, it would just die after struggling to get
to 500 rpm. I tried it again, only this time revving the
engine for a couple seconds and then letting off. It
idled around 700 and worked fine the rest of the drive.
Now, it just sounds absolutely awful, like it's going to
die whenever I have to stop and idle in traffic. Driving
however, it works fine. It has shut off once more since
the first one, but it turned over and drove dine after. It
did finally pop up a code, p102 and p0171. The guy at
AutoZone said that I probably should just clean the
mass air sensor, so I removed that today, used the
recommended cleaner on it, and replaced. Cranked
the car over, and it still struggles to stay even at 500
rpm-- UNLESS the A/C or heater is turned at full blast.
Then it stays just about at 700-900 rpm with little
trouble. Any ideas?