thermostat problem?

I have a mint condition, 1986 SR5 4WD p/u. It's been garage kept and hasn't been run for the last couple of years. This spring I started running it again and ran it all summer and fall with zero problems. Yesterday, here in the cold northeast, the temps were in the 30's. On my way to work (about 8 miles), the truck never really got up to temperature (the needle was barely off the peg). Normally, it is in the mid range. The heater blew warm (but not hot) air. When I left for the day, the outside temp was ~ 55F. The truck ran normal and the temp got up to the mid range. That evening, I had some errands to run; temps were in the 40s. Same thing happened; never really got up to temp. I stopped in a grocery store for ~ 10 minutes and when I came out and started the truck, the gauge was up higher, but not in the 'normal' range. As I drove it the first couple of miles, the needle went back down instead of up. But, after about 5 or 6 miles, it immediately went up tot eh mid range of the gauge and acted fine; the heater blew hot air. This morning, the same thing happened; didn't get up to normal temp but the heater blew warm air. But, when i got close to work, it acted 'normal' and got back up to proper temp.

So, what's wrong.....thermostat....that's what I assume. But could it be a sending unit or just a slug of air? Any thoughts? Thanks.

Nothing sounds that bad.
Driving only 8 mi may not be enough time to warm up in cold weather, in warmer weather it would be OK.
You could see spikes when the t-stat opens or closes.
If it becomes a problem I'd start with the t-stat, simple and cheap

you can always use the farther north trick and partially block the radiator with cardboard or something else easily removable.