Mine started doing that just before a scheduled coolant change. While I was there I changed the plastic thermostat housing with a metal one. While I was there I changed the thermostat. WHen I saw the old thermostat I found out why it was overheating when it wan not moving: the thermostat had physically broken, I'm lucky the water pump didn't start chewing on the parts
IMHO thermostats should be regularly changed every 5 years or so. How old is yours?
5 speed '91 190E 2.6 320,000 mi. (new car, fast, smooth as silk six, couldn't find any more Peugeots)
5 speed '85 Peugeot 505 2.5l Turbo Diesel 266,000 mi. (old car, fast for a diesel, had 2 others)
5 speed '01 Jetta V6 (new wifes car, pretty quick)
5 speed '85 Peugeot 505 2.2l Turbo Gas 197,000 mi. (wifes car, faster, sadly gone just short of 200k )
5 speed '83 Yamaha 750 Maxim 14,000 mi. (fastest)
0 speed 4' x 8' 1800 lb Harbor Freight utility trailer (only as fast as what's pulling it)