Over heating puzzler

[69stepside4x4]

I have recently had a problem with my '72 K5 temp gauge moving quickly to the hot side of the dial and don't really know why. As some of you may know from former postings, I recently returned after a year-and-a-half living abroad, so my truck was stored for a while.

Although the gauge doesn't move over immediatly to the the hot, it does shoot over after only running 5-10 minutes and then will back down (just a little) when I am at a traffic light?????? I thougt it might be a defective thermostat, so I replaced it today, that's when a bigger problem occurred. After installing the same temp thermo, the temperature shot up quicker than before and went even further past the hot marker (but not as if there was a guage short or anything, it just started moving up as I drove around the neighborhood and was up high within 3-5 minutes). I am stumped. Help please!

Some additional background is that I have just had the TH350 rebuilt and installed by a quality shop, but this problem was noticed before I had this work done, so I don't think it's the tranny.