Hi,
the thermostat gets stuck at a certain position, usually. Commonly, it gets stuck in open position, leading to an engine staying too cold during winter months or requiring too much time to become warm.
I've never heard of a thermostat suddenly closing during driving and staying there... but I've had the same symptom as you describe it once in winter, when I had too less G12+ in the cooling circuit, resulting in a frozen radiator. This in turn resulted in >110°C and, consequently, in a blow off of the overload pressure.
All this still sounds more like a clogged radiator to me, but give the thermostat a try.
Bastian
