Volkswagen Repair: Check Engine Light Issues, gas mileage, maintenance history

Question
Just bought a used 2000 Jetta GLS from a private owner with 52k miles on it. It has a manual transmission and a week after  I took it home, the check engine light came on. I took it to autozone and the code said that it was the 02 sensor (specifically the one before the catalytic converter). I waited a while before I decided to do any repairs and a few days later the light went off and I had better gas mileage. Things seemed to be getting better. I was told that 02 sensors had the ability to re-adjust themselves,  so I assumed that the problem was no big deal until today when it came back on the freeway. I'm guessing that because it's got a manual transmission, my driving style must be different from the previous owner? It's not like I drag race the damn thing, lol. I was going 60 and needed to pick up speed, reached 80 on the freeway and got to where I was going. When I turned the car off and on again, the light appeared. The exhaust smells really rich with gas and the idle is a bit unsteady. Not too much that it seems like it's about to die, but enough for me to notice it. Shifting and everything is fine... just gas mileage is horrible right now. Should I wait it out for a few days again to see if it "adjusts" itself again? I want to avoid spending a lot to have the 02 sensor replaced if it doesn't need to be. What should I do? Oh god please don't tell me this car is going to fall apart on me... I heard some bad reviews about Jettas (a little late, eh?) AFTER I bought it. As far as maintenance records go, i'm not too sure. The guy I bought it from was the 2nd owner and he's in the Coast Guard, away at school for a month somewhere so I can't get a hold of him right now for the maintenance history. Anyway, sorry that was such a long question, I was trying to be as detailed as possible! Thanks!

Julia

Answer
Thank you Julia;
You provided a complete, and well thought out description.  Your question is short, compared to some of my answers.  Don't worry about records, and/or past maintenance, what's done is done.  Others peoples opinions are just that.  Volkswagens are, for the most part very good, very reliable cars.  You are the owner now, so just get the oil changed...A LOT!!!  Do it often, like about every 2000-2500 miles, and just don't miss.  Your car will be as reliable to you, as you committed to it's maintenance.  Also, don't get the cheapest oil change...you get what you pay for, and the cheapest places hire the cheapest labor.  The cheapest labor is PRONE to mistakes...take it to the dealer for a used car inspection, right away.  This will require that you pay for 1 hour of labor, but you are NOT obligated further.  Let them tell you that your car qualifies for a Volkswagen sponsored "good will" campaign that has been extended from it's initial expiration date of last year.  This campaign deals with the catalytic converter, and you should be offered this service free of charge.  In other words it should be covered under a warranty replacement program.
Now let's tackle the specifics of your engine, and the way your 2000 MY 2.0 litre 4 cylinder operates.  Most 2000 model year 2.0 litre engines are "AEG."  This is the engine code, and describes the ONLY engine in "MkIV" Jettas that has a throttle cable.  ALL other engines have a couple of sensors that deliver the air to mix with the fuel.  I like AEG engines because they demonstrate fewer issues than most, and they provide a physical connection between the driver, and the engine.  All other Volkswagens are "fly by wire" accelerator design.
The most common failure is the "Mass Air Flow Meter."  It demands the necessary fuel from the "ECM," and finally the fuel injectors.  If the "MAF" sensor fails...fuel "trim" goes way out of whack!  One of the most common reasons these fail is due to a huge variation in Voltage, and/or Current from the battery.  I would recommend a new good quality replacement battery, too.  A complete charging system performance diagnosis should be included with your used car inspection, at no additional charge.  The oxygen sensors are resilient, and can withstand alot, but they CANNOT withstand continuous fuel contamination, so they should not be replaced with the Catalyst...unless one thing happens, and that is IF the "Cat." is replaced by the campaign I mentioned earlier, and IF the "readiness" code will not reset.  "Readiness" code is a Volkswagen/Audi term for testing all of the emission control equipment on their cars.
I hope this helps.  Good luck, Julia.