Since my last post on this, I've been driving the truck as much as possible. Errands around town, work and back, longer trips, etc. and driving it like I stole it. Checked fluids today, and nothing was low. I think (hope) the earlier theory about some air finding it's way out might be the winner.

I do think that the thermostat was sticking for awhile there... but the past couple of days, no erratic temp guage. In fact, I let her idle this afternoon with the radiator cap off, and no fountain!

Probably too soon to be sure, but I do feel better. I'll probably end up putting a new thermostat in just to be safe...

 

Don't get the cheap one, they stick right out of the box. Get the good one and test it too.