318 overheating
Your fuel gauge and temp gauge are on the same circuitry, but all of the above is worth looking into.
Meaning that if the DVR is acting up, it will affect BOTH gauges. But if only one gauge is acting up, then it may be telling the truth.
The DVR is supposed to put out a 50% duty-cycle pulsed 12 volts. Which is supposed to generate an average 6 or 7 volts power supply to those two gauges. But the gauges are so slow to react that yo never see the pulsing.
The gauges work by grounding the other side thru the senders. With calibrated points that correspond to the markings on the gauge faces.
But the temp sensor HAS to be in the moving coolant to have a half a chance at being accurate. If the sender is in air, or in a stagnant pool, it will read low. If both gauges read high, then the VR is outputting too high. If ONLY the temp gauge is high, then either it is telling the truth or close to it; or the sender is fubarred. The sender can be ohmed out and compared to specs, or simply replaced. But they very rarely go bad.
If the DVR is intermittently showing erroneous readings, you can bet it's because the supply voltage throughout the run circuit is fluctuating. It could be the charging system, or the fusible link, the bulkhead connector, the ignition switch, or even the amp-gauge.