Isuzu I-Mark Cold Start Problems

I've got a 1988 Isuzu I-Mark that has a cold weather starting problem.
When it's cold outside, it will start up with a really fast idle (2000 RPM) for about 3 - 5 seconds, slow down to about 1500 rpm's for a while (10
seconds perhaps) then start bouncing down to around 500-600 RPMS, almost stalling, then back up to around 1200-1500 RPM's, and repeat this process maybe 10 - 20 times before the engine seems to find a good place to idle at. When the car is warmed up, the idle is rock solid at around 900 RPM's.
Last week, on a really cold morning, it would not start at all. The starter would turn, the engine would start for about 1 second then sputter out and die right away. After about 2 -3 hours later, it started like normal (normal with the jumpy RPM's).
I'm thinking engine coolant temp sensor... can anyone please help me?

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