Volkswagen: rough idle, mass air flow meter, mass air flow

Question
I have a 2001 vw jetta with 2.0 and auto trans i have been  having trouble with it idling rough and black exhaust it seems to get alittle better after it runs a while but it runs rough in traffic too. i had it in the dealership and they changed th rear o2 sensor and said it needed a tune up which i did myself after putting  in new plugs it ran great for a day or two but is now back to the same. i had auto zone runthe scan on it and theirs came up with multiple cylinder missfire and 02 sensor this tells me nothing about what is causing the problem can you help me?

Answer
it sounds like one of few comon symptoms / problems. On nearly all volkswagons, the coolant temp sensor can cause running problems as it gives the wrong signal causing the car to run rich or lean. also, with similar effects, the maf (mass air flow) meter causes the same issues. it sounds more likely to be maf sensor at fault. this is the part fastened to the air box with a wiring plug on it. the part number will begin with "06A or 074". hope this helps.