ANY deviation in that 12 o’clock strait up temp gauge position after it’s warm is telling you there is a problem. Here are a few things to check. The fact that the gauge is reading cooler makes me thing you have a thermostat that is not closing, but there are many things to look into.

-Expansion tank, look for cracks
- Check Radiator cap for leaks, if it’s original replace it, it’s probably not holding pressure any longer. Change your Oil cap at the same time, they need to hold pressure also.
- Check around the thermostat housing for any indication of leaking.
- check fan clutch for resistance, it should be firm but not jerky when you move it (engine off obviously).
- use a mirror to look under the water pump, that’s where leaks usually start.
-check the heater valves for leaking, they can suck air.
-check system for air at the bleeder

Look for any dry coolent it will be white chalky deposits

If you in the 100K range a water pump, thermostat and fan clutch are not out of the question to replace.