Crazy overheating problem

[jmasco]

Can anybody help? I have a 1982 5.0. Since the cool weather arrived, I had to turn on the heat. When I turned on the heater, the temp. gauge started to climb like mad & the air out of the vents is not all that hot. I recently have installed a new thermostat, temp. sender & a fan clutch. When you turn the heat off, the gauge goes back to normal - I am puzzled. I tried to turn on the AC to see what happened, nice & cool 59 degree air out of the vent - temp gauge normal.

Anybody have any ideas to set me on the path to cozy warmth? I know usually when you put the heat on, the temp gauge goes down - this is why it's got me scratching my head. I would really appreciate any helpful information. Thanks!