Chrysler T&C with fluctuating temp. guage and heat

I have a 02 Chrysler T&C limited 3.8L with 176K.

Earlier this week in the morning when I put the heat on after about 5 minutes or a couple miles (outside temp about 40 degrees), I noticed the heater was blowing cold air even though the temp gauge was moving up and was usually starting to blow warm air by then. Once the temp gauge reached it’s normal position, The heat would start. At first I thought it was a blend door not operating properly.

Tonight I went out with the same scenario as in the morning, I got about 2 miles out and noticed the temp gauge was well above center. I kept the heat on and after getting on the highway the temp gauge centered again.

Any ideas to what is causing this?

Make sure the coolant level is full. If that is ok then have a block check done to see if exhaust gases are getting into the coolant due to a head gasket leak.