Seb;

Those symptoms are pretty strange...so I don't have any immediate bright ideas...but hot or cold weather shouldn't make that much difference as far as the cooling system goes...that system is typically operating at more than a hundred degrees above outside ambient, so during cold, one would expect the cooling ability of the radiator to be even better (bigger temperature difference) than during hot weather conditions, not worse...

I suggest you do a couple of things...test the tempgauge accuracy to determine if those high temp readings are really "in the red zone" and something to be concerned about...upgrade your cooling system to be a "sealed type", so that you dont loose those 1-2 liters of coolant every time (hold rad cap off seat, and add a tube and an overflow bottle to catch and return coolant)...determine if there is good flow in the system (when the indicator shows hot, carefully feel the bottom and top of radiator...if cold, flow, which is enabled by t-stat and pushed by pump is suspect)...a dislodged coolant distribution tube in the head might cause flow problems and/or localized boiling near temp sensor...

You did not state condition of heater valve...if you open this when indicator is in the "red zone" (set heater to hot, and shed more BTUs from cooling system) does this help?

I suspect some sort of flow issue, but that statement alone doesn't help much.

Prost Neu Jahr!