[unstable bob gable]

This is one a the strangest situations I've ever had w/ a car...

OK, 03 Cavalier, 2.2 Ecotec. Approx 26K miles on it, stock except for CAI and plugs. I was driving it home from work on Xmas day, and it started to cut out on me. Engine didn't want to rev, was sputtering, and "Traction Off" light came on. I limped it off the Parkway, and was all set to call AAA. But I let it run for a little bit, and I was able to get it up and running to highway speeds. So, I thought maybe it was a "random" incident w/ bad gas or something. I was at about 1/2 a tank, so I filled up w/ premium on the Parkway, and all seemed fine. OK, was fine for a couple of days, but started to do the same thing again when I got to around half a tank...hmmm, pattern showing here? I limped it to work, and went out on my lunch break and fired it up. Seemed to run OK, and "Traction Off" light went out. So, I drove it to Jersey City and filled up, and it ran fine all the way home. I figured I'd try for a "quick fix," so I replaced the fuel filter, and also replaced the ignition module in case it was "spark" related. Welp, the car ran fine again until, you guessed it, I got to half a tank. I pulled off, filled it up, and it was fine. This time I figured I'd run it close to half a tank, then add some alcohol to dry up any water that may have accumulated in tank. Well, on the way to work I got to half a tank, so I added the alcohol, and got it to work w/o any incident. On the way home it started acting up again, and this time it threw a code :
P0171....System too Lean (Bank 1). I was able to limp it off the Parkway, and I stopped and filled it up, and cleared the code. So, now I'm stumped....it only acts up @ half a tank, and if I had water or contaminated fuel in the tank I would think it would have gotten mixed up with the fresh gas and or alcohol by now and have gotten run through. I guess I can just continue to fill up before half a tank and live with this problem forever, but I'd rather take care of it...any thoughts?