For me it was a radically warped head. Get a combustion leak tester (exhaust gas tester), and test your coolant. It was super obvious on ours. I considered reworking the head, but both my machinist and VW staunchly recommend *not* milling one of these heads. I ended up finding a 20k mile motor locally for about $1200 and swapped it in. It's actually easier to swap the entire engine than pull the head in the car.
If you need parts, let me know. I still have my previous motor in the garage that I need to offload. I'd let it go for shipping, if someone wants it. I can even pallet it up.