This car runs fine. When starting up from cold, temp gage rises like you would expect as the car warms up. Then suddenly, when it reaches operating temperature, the gage drops QUICKLY into the cold portion of the scale. This takes place in about 2 seconds. Radiator is full. Thermostat has been replaced several momths ago. You'd think if there was an air bubble, it would eventally disolve into the anti-freeze. Any theories?

Forgot to mention 2.2L

your coolant gauge sensor is bad, had the same thing to happen to me on my '92 Spirit. It's located near the thermostat housing, with a purple/yellow wire going to it. It's only 4 bucks to replace.

your coolant gauge sensor is bad, had the same thing to happen to me on my '92 Spirit. It's located near the thermostat housing, with a purple/yellow wire going to it. It's only 4 bucks to replace.

You know what it was? The contacts on the coolant sensor were corroded. On-and-off a couple times and PRESTO, it is fixed!!!