Jeep Repair: 03 Liberty constant problems, how a car works, 7th time

Question
I have an '03 Liberty that has given me problems for the past 3 years.  I have installed a new transmission, rear differential, complete A/C system, E-brake system, 4-wheel drive wiring, water pump (x2), serpentine belt (x2), and back wiper motor in this time period.  My check engine light came on recently again (for the 7th time!) and I took it to AutoZone who told me it was the O2 sensor.  As soon as they pulled their diagnostic tool out the light went off and stayed off.  After 2 weeks it came on again when the temp dropped to -30 F outside (live in MN).  It was on for 3 days and then went off again.  Now, 5 days later, the Jeep doesn't want to accelerate without having the pedal to the floor, it feels like I'm dragging weights under my car, and it takes muscle power to turn and even keep the wheel straight.  Is this still potentially the O2 sensor and the light went off for who knows why, or is this a much bigger deal?  I am within 20 miles of hitting 200,000 so it is well beyond its years...I just don't want to get stuck on a long drive.  ANY help would be great!

Answer
first of all   no matter what the code says  it does not always mean  that is the prob.
If this was the case there would be no need for mechanics  any one can change a part but a real tech knows how a car works and how a system is suppose to be acting. This is what U pay for.

What is the code that autozone gave u?