Jetta overheats

It sounds to me like the water isn't circulating, due to either a defective circulating pump or a slack belt failing to drive it. You should be able to check for circulation by removing the coolant / expansion tank filler cap when the engine is cold and looking inside, where there should be a little stream of water coming through that thin pipe near the top and going down into the tank.

Hope this helps you a bit. If it does, a "Fixya" rating would be much appreciated. If it doesn't
help you, I'd be grateful if you didn't bother with a rating at present - they can't be increased later if I do manage to solve your problem! Just come back and tell me what you found. Cheers, D