'89 323 runaway idle on cold mornings

[tdinning]

This is an '89 hatchback with 5 speed, non-turbo.
Starts fine, but after a few seconds, the fast idle is going so high it seems as if the engine will disintegrate. Engine temp doesn't seem to slow it down, as it's still way too fast even once the thermostat opens. Once the thermostat is open and it's driven 10-15 minutes longer it slows down to near-normal, and then I don't have the problem the rest of the day. It was about 20 deg. F. here this AM. Drove this car all summer and Fall with this not happening, so it seems to be related to outside temp. The car has 295k miles and is otherwise a strong runner. It's "The little car that could." Any help greatly appreciated.