First Things First:
Those temp gauges with the compensator boards are the worst.

You should first ensure there is coolant in the system (filled properly).
If so, bypass the compensator board.
This will likely cure all apparent problems.
If still showing high or low, go after the rest of the system - thermostat, radiator flush, you name it.

Probably the best instructions for the bypass are at ipd's site (under cooling system), see their posted instructions. [ ipdusa.com ] Solder a jumper wire, or find my earlier posts on this and slip a 16-gauge wire (butt-cut) onto the proper pins. Works great.

I've spent countless hours and way too much money replacing cooling system components and thus coolant repeatedly, all due to nutty compensator boards.

I also replaced the compensator boards more than once.
Did not help much.

Once the board is bypassed, you'll get good information from the gauge, instead of disturbing misinformation.