Low coolant temperature and engine management.

Get the code pulled for us. It is possible but we need the code.

Roy

Yes, probably:

P1296 Cooling system malfunction... or similar, error in mapped cooling system performance.

Thermostats open too soon, causing low engine temp.

Trouble code will set if engine temp drops below 81c after warm-up (87c+) has occurred.

Remove and replace thermostat.

Other possibility is coolant temp sensor malfunction.

You can eliminate one by data logging the coolant temp via scan tool... a sudden big drop in temp is a sensor problem. a steady
drop while cruising is a thermostat issue.

Here's a video of me changing a thermostat on a New Beetle 1.8T engine... I assume your TT is a 1.8T.

http://www.youtube.com/watch?v=QW8gOHVx2mI

Thomas