New overheating problem

[Ivan141]

Guys, I need to drag this thread up from the cemetary.. My car is having overheating problems that I cant put my finger on lately.

It will not loose coolant as long as I drive normally, but after about 10 minutes of traffic jam it will start overheating, and looseing coolant.

I cant spot any serious leak, no puddles under the car, nothing obvious when running at idle. Turning the heater up to max doesnt seem to help either.
Today it happened again, so I turned the engine off as much as possible , but still it would shoot right up again after a short bit of crawling speed.
It ran normal temperatures once I got moving normally again.
I checked the coolant reservoir as soon as I got home, it was pretty damn high. I dont notice any oil or bubbles in the coolant though.

Any ideas? Thermostat is about 2 years old, waterpump is new and on the car since last summer.
I'm thinking it's either the thermostat, or maybe the radiator cap...possibly BHG, but not likely, it doesnt smoke much either.