'92 XFi: intermittent high idle + high temp

I have an intermittent condition that I find very puzzling. It just started this winter (pretty cold weather here in MI).
I'm driving along and all of a sudden I notice the coolant temp gauge (which is always dead nuts in the middle) climbing up very close to the "too hot" area. I turn on the heat full blast and it goes back to the very middle and stays there (the heater core on this car must be as big as the radiator!).
The first time it happened I was on the freeway. The only symptom was the temperature, so I told myself I need a thermostat.
But subsequent occurences (not on the freeway) have been accompanied by a very high idle speed and/or a rough idle, which goes back down to normal after I bring the coolant temp down.
Fault code = 12 (no codes stored).
Any ideas?

Thanks in advance.

Might be an air bubble in the system, but most likely thermostat. Replace the thermostat, jack up the front of the car so the radiator cap is the highest point, let it run for half an hour with the cap off. You'll need a pan under the radiator, keep refilling it as it runs over.

I have a new thermostat, and I'm going to put it in.
Unlike the one in there now, I got this one from the GM dealer, and it has the OEM jiggle pin to let it de-aerate when filling. I almost overheated the car when I put in the one that's in it now, because of an air pocket. After that, I figured out that removing the temp sensor from the pipe behind the engine that leads to the 'stat will de-aerate it just fine, if you have one of those aftermarket 'stats without the pin.
I'm still not sure the new 'stat will solve this, though. Two days ago, after driving 25 miles in blizzard conditions, the gauge drooped way down to the "too cold" area. This time, turning the heat off brought the temperature back to normal. I guess maybe it is the thermostat. I'll find out.
BTW, the fast idle condition has only accompanied the high temp condition on maybe two occasions. The other times it's just been the temperature. This last time, when it went cold (when I would expect a high idle), the idle was normal.
After correcting each of these temperature excursions with the heater, the problem seems to fix itself for a while.

I live in a part of Canada that experiences half of it's year in sub zero temps. I use a "winter" thermostat at this time of year. Many people here also have a "bra" for the front of the car to reduce the amount of cold air blowing across the radiator. The winter thermostat (185 degree) keeps the coolant in the block for longer before opening up. I believe the difference between a summer and winter thermostat is only 10 degrees, but enough to make a difference. You may need to have one of these for winter.

Jai

I flushed & filled the coolant and put in the GM dealer (Suzuki part) thermostat. Hopefully that will fix the temperature problem.
I think the high idle is unrelated; it occurs mostly on its own without a temperature condition. It may have something to do with the broken bracket I just noticed:
There is a vacuum biscuit right on top of the engine that is connected by vacuum hoses to the EGR actuator among other things. It is mounted to a bracket that attaches to the intake manifold. The bracket cracked and the whole biscuit thing is now just suspended by the vacuum hoses, shaking around to beat the band.
I have to find a replacement bracket. Any sugestions where to look?

I think I have blockage in my heater core.
The other day while driving home in 5 deg. F weather, it all of a sudden started blowing cold air. I looked at the temp gauge and it was climbing toward the H. I'm guessing the flow through the heater had reduced but the t'stat's reaction time hadn't yet elapsed.
Then the heat came back and the temp gauge went back to normal.
I'm going to flush the heater core and also inspect the heater hoses, as soon as it thaws out here in MI. I'm thinking a blob of something is bouncing around and occasionally getting stuck; maybe some rubber hanging loose on the inside of a hose...