1996 Chevy Cavalier uncomen problem

Heater problem
1996 Chevy Cavalier 4 cyl Front Wheel Drive Automatic 150 p miles

Thermistate was replace head gasket presher tested fine heater core presher tested fine no loss of fuild all fuild fine run great other then my problem the temp gage works fine untill I put the heat on will go to cold

C (_?_______n_________) H this is were it at in drive heats on.

C (___?______n__________) H this is were it is in park

now it has to do with the cold weather to because today I tock it to Menikey and it did not do what it has been doing but did go to cold just did not drop like it has been doing slowly went down towards cold with the heat on will not go back up untill I shut the heat off please any more ideas