Oxygen Sensor Problems

Hi I have a 2000 Jetta GLS 2.0 Liter 4 Cylinder and every year around december/january my check engine light comes on and my exhaust pipe starts spitting out black stuff onto my driveway(possibly carbon) I take it to wherever to get scanned and it comes back as the oxygen sensors need replacing.....This has been going on for 4 years running now....Has anyone else had problems with this and/or does anyone know what causes this to keep happening? Thanks:banghead:

Mine has just started doing this. It will start up and the idle will move between 500 - 750 rpms, and some time it will stall out, I runs fine driving it, but idleing it has problems, was told it was carbon buildup.