Seb;

After speaking to an aquaintance about your symptoms, he had a similar experience on his 122...while an incorrect thermostat was installed. The type with no small hole in the plate of the t-stat. He said that without it, the t-stat will not allow a small amount of flow, so will not be surrounded by coolant (sort of vapour locked) and wont sens temp correctly to open.

He would see temp climb, and after manually squeezing the top rad hose to get some hot coolant to the t-stat which would cause it to open, the temp would go down and stay in the normal range until the next time the car was started from a cold start.

...of course that doesn't explain why the problem only occurs during cold weather...

Cheers