Why is my 1998 Volvo S70 cutting off?

[JohnMichaelMusic]

I just bought a 1998 Volvo S70 (base) from my job (volvo dealership) The car has 220,000 miles on it, but was well kept by an elderly lady who serviced it fully. The volvo mechanics checked it out and said it was in good shape. It drives great normally. I've noticed that every day the engine will just cut off once or twice while driving. It is always when I am at a low speed or low RPM or parked. If I mash the gas it will hold it off for a little, but eventually it will cut off. Sometimes it cuts right back on, but sometimes it doesn't. When it does cut back on it drives fine. It's weird. I had the mechanics check it out (not thoroughly yet) and hook up a computer to check for errors, but they didn't come up with any. I took it to Autozone today after it happenned again and they got a PO171 code to come up. It said "The PCM has determined that during testing, the fuel system for bank 1 was too lean). Does anyone know what might be the problem???The car drives great otherwise.