2003 Ford Focus has continuous po171 code accompanied by the check engine light every 2 weeks?

2003 Ford Focus has Po171 code? I have replaced spark plugs, wires, oxygen sensor, MAF...and the check engine light continues to pop back on after about 2 weeks with the same stalling effect...the only weird thing is...if I turn my air conditioner on..the car's idle will smooth out and not stall. I have taken it to a shop like 6 times in a 3 month period..before taking it in..we changed spark plugs, wires, oxygen sensor, MAF and oil...then the check engine light kept coming on so I just took to a shop in frustration. So they of course changed the spark plus and wires again..I paid for this and told them that was not the problem so I will be seeing you again in a couple of weeks when the light pops back on..well that was 3 months ago and like clockwork I take back every 2 weeks when the light comes back on..they can not figure out the problem..I think they have changed the oxygen senor twice..the car does not over heat or anything like that..just stalls without the air on and the light repeatedly come on and every time I take to Autozone it is the same code po171...there has to be one culprit that is causing this..please help!!!!