Hi,
Sorry for the delay in replying, a very bad week at work.
The thing in the cabin wouldn't be the problem. It only deals with the engine when it is cold and the throttle is in the idle position. There is a tiny dip switch in the throttle body that tells the ecu when the engine should be at idle. It probably also trips the idle control stabilisation unit as well.
Its not that.
It could be the crystal doobry whatsit (richandhazel will tell you its real name), which can be found under the drivers wiper. But i would doubt it from what you've said. To test - find a mate with a GTI and try his.
I can ask around, but could you describe a bit more what happens.
Cheers,
Tin