For the cost i would try a new thermostat and have it installed somewhere else or do it yourself to make sure that it is put in correctly . If you think it might be a heater core ( idont think it is ) get your system pressure checked most shops wont charge you to do this . good luck

 

you can check the proper temp of a t stat by putting it in a pan with water and a meat thermometer and heating it up. Dont let the meat thermometer touch the stat or the pan. when the stat pops open, check the temp on the thermometer to see if it's close. If it's off by +/- 5* get a new one. Stant is a decent brand.

 

Well now I don't know. Today it seemed to warm up just fine and the gauge was a tad higher. It was about freezing outside. I guess I'm worrying over nothing. Still don't see why it would fluctuate so much. Maybe there was a little air in it or the stat had to break in. I wish I had the guts to tear into the engine to check the stat in water but can't risk screwing something up. I need the truck every day.