98 Grand Prix SE 3.1 Heat and Idle problems

I have a 98 Grand Prix SE 3.1 litre engine. 178,000 kms on it. I recently changed my coolant and flushed my rad. My coolant level is good. I checked and the coolant ratio is fine as well. I changed my thermostat last year. Last night (first cold night since I changed coolant and required heat) I noticed that my heat was blowing cool air. The temp gauge was going up and down. Heat came on sporadically and the temp gauge would drop.

I was thinking I might have an air lock or need a new thermostat again. Someone told me there is a bleeding valve on these engines. I did not bleed using this when I changed the coolant. I stepped on the gas when in park and revved the engine to see if this would cause the temp gauge to come down. It did. However the RPM would go up to about 4500 RPM’s and then the tach would go up and down from about 3800-4500 RPM’s and not any higher. The engine started going crazy revving up and down.

Any idea what could be causing this? Sensor, Air lock, Thermostat? Something completely different? When I drove it seemed fine. Accelerated fine. It only did this in Park. Will gladly donate for some help. :D