weird idle problem

I have a 98 Jetta GLS 2.0 5speed. I have a Check engine code P0442 which I think I have traced to the gas cap. When I come to a complete stop, the engine revs to 2000 rpms then drops to around 1500 rpms then back up to 2000 then... well you get the idea. Eventually it does taper off to around 1000, but it has this cycle about 5 or 6 times. I was thinking O2 or maybe MAF. Any guesses?

Thanks

Hunter