292 water temp problem.

[factoryorange3]

I have a 79 Chevy k10 that I swapped a mid 60's 4.8 292 motor into. Have been driving truck for about a year just recently put a rebuilt head onto motor (new water pump, thermostat) I am experiencing inconsistencies in water temperature. I will start truck in morning to let warm up .(temps been below zero lately) I watch temp gauge slowly rise and keep rising almost to red before gauge slowly drops again to operating temp. If I let truck idle gauge will go up again dangerously high until I throttle engine up in order for temp to go down. The heater in the truck blows good heat when at operating temp but when gauge rises I lose heat in cab until I throtle motor up and gauge drops. When I did the head work I had radiator cleaned and flows great. Yesterday put another new thermalstat in truck but did the same thing this am. Any ideas of what might be happening would be great!! Thank you.