I had the exact same problem, albeit with a Polo, for the past couple of days and can point you in the direction of a few things to try out first because ultimately they are both VWs! 1. Turn the ignition to the point just before engaging the starter motor and leave it there for a good 30 seconds before turning on the engine. This supposedly tells the ECU to check some boring things in some cases, although didn't work for me. 2. Try disconnecting the battery and leaving it disconnected for an hour or so. Write down any key codes you need for your radio first! This should reset the ECU and when you turn the car back on it should re-calibrate the throttle body to the right idle speed. 3. Clean the throttle body with some good carb cleaner, rather than me type it out you can look elsewhere on the net for how to do this on a golf. 4. Change the temp sender - again do a google search to find out how. If you do them in that order you should hopefully cure the problem before having to spend any money!