I just finally got my entire cooling system done... apparently no more leaks, no more cool-running thermostat.

And today, I found that my temperature gauge would not ever reach full temp. I had been losing coolant through a radiator hose fitting, but I replaced the coolant and the hose clamp and daubed on a tiny amount of non-hardening "gasket maker" - I accidentally overfilled the system, and the "fixed" hose connection leaked, but not so today.

I then noticed that when I got back into the car from stopping for a delivery, lunch, whatever after 5-45 minutes, the needle started at normal temp, then gradually dropped in temp...
Eventually, I was sitting in the car with it in KP2 and the temp gauge started twitching minorly - not oscillating.

My aux fan is not stuck on.
Should I be looking at the temp gauge or the cooling system? I believe that the dash gauge is fed with a separate temp-sensor. Could this also be a problem? If so, how do I determine push-in vs. Screw in aside from partially removing it?

TIA,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond