Heater problem
2002 Buick Rendezvous All Wheel Drive Automatic 86000 miles

Not all the time, but 90% of the time, my heater blows cold air when it is set on high.
My blower is strong, my car is running for more than 30 minutes, but it's still cold air.
I have also noticed that when my heater is blowing cold air, my engine thermometer is to the 3/4 mark some of the times.
If I need to replace the heater thermostat, how do I find/do that?

UPDATE!! Several hours after I posted this message, my car "overheated" within 3 miles of leaving work. All my fluids were full, clean and after I let the car sit for 30 minutes, I was able to drive it home all while the temperature gauge fluctuated rapidly from cool to hot.
The shop found that both gaskets on my intake manifold were leaking. This is a common problem in this year of GM engines and they won't do anything about it even though they are very aware.
One of the leaking gaskets was very close to my temperature gauge and as the coolant shot out sparatically, this would cause my temperature gauge to fluctuate from hot to cold, causing my car to think it was overheating. If you have a GM engine model ranging from 2000 to 2004, check for these major problems.

You may possibly be low on water. If so, the heater core is empty and therefore, blows cold air. This could also explain the higher engine temps. Check the coolant level first! If it's ok, we'll start checking the operation of the heater and controls, such as the blend door motors, electronics, etc...