Slant Temperature

Your water temp and gas gauges run off a dash-gauge regulator
The regulator is nothing but a set of mini points with one contact on a bi-metallic arm. As the electrons flow thru the arm, the arm heats up and breaks the circuit. As it cools, it re-establishes contact. It is designed to have a 50% duty cycle, so if it sees 12 volts , then it will regulate to 6 volts. But if it sees 14.5 volts, then it will read 50% of that which is 7.25 volts.
Ergo;
If the system voltage is too high, then BOTH gauges will read high. If the system voltage is low, then the gauges will read low.
In the pic of the gauges , the alternator needle is way over on the right, indicating a possible high charging rate, which can be measured as a high voltage at the battery.

Stabilize your charging system voltage, and your "overheating" problems will be cured.