As this has long been discussed, I've found the fault, if it's not the temp sender, which mine wasn't.

I decided that due to it's behaviour and the fact that it is digitaly driven by a stepper motor, must be some dry joints. It takes in the pulses from the sender, works out how much the temperature has gone up or down since the last sample and sends out a few pulses to the gauge to move it to the new position.

So I had the cluster out. Audiworld has a tech article on that, but very simple. Once the temp gauge was out I noticed a dry joint. Other joints looked suspect. I soldered but did not test it. I took the back of the mechanism as I could see other joints. It's not a good idea to do this as it doesn't go back as was due to it's manufacture.

I sourced a new cluster from a breaker. £40 and got the whole cluster inc DIS. I swapped the temp gauge over and all is fine.

If anyone has this problem, check the back of the temp gauge for dry joints first and test. I've read many folk get a new cluster from Audi for £350 to £450 pounds. Joke.

Cheers
Marco