Help, I keep replacing the same parts

I am the tired owner of a 2001 Jetta. We bought the car, brand new with 80 miles on it. It currently has 73000. Since we have owned the car we have had the O2 sensors replaced 4 times, the mass air flow sensor 2 times, every time from my authorized VW dealer where I bought the car. I am sick of paying this much money for mechanics who can't tell me why either one is going out. The vehicle gets normal wear and tear and is maintained. The most recent problem is the fromt O2 sensor (which I already replaced in 03/05). The nasty mechanic told me the car has too much carbon build up, and the only way we will solve our problems is to "de-carbon" the car for the price of $300+. I think he's pulling my leg, if this is the problem why hasn't anyone ever discovered this problem before. Can anyone tell me if this makes sense, also how will I be able to tell that he actually did something. Silly me but for $300 I like to see proof. He also said the MAF sensor will need to be replaced, as it is going bad (how convient that we are 3000 miles over the extended warranty). Is this a common problem with all Jetta's or did I buy a lemon. Does any of this make sense to anyone, I don't know much about cars other then I am sick of them!!!

sounds like you never had the wa recall performed... its for 02 sensor calibration of the ecm. also carbon buildup is a common problem on 2.0. a 6.00 can of intake system cleaner , 8.00 can of seafoam, and 20min of time can help that.