Replace Engine Coolant Temp Sensor or ECM?

[bybyc5]

Well there is only 3 things it could be..The sensor, the wiring, or the PCM. You have ruled out the coolant sensor. The harness can be the next easiest ( well cheapest anyway ) thing to verify.
Odd that it jumps to a high temperature, indicating a short to ground somewhere in that circuit.
If you can individually isolate the circuit wires and test them from the PCM connector to the Coolant sensor connector for continuity, with no pinched wires intermittently shorting to ground somewhere, then it must be inside the PCM.

Sometimes it's easier to overlay temporary wires from connector to connector if the circuit continuity test does show a problem and the harness is long and buried or routed in such a way that exposing the problem is not easily done. It's hard to visually inspect every inch of the suspected wire for failures..shorts or open circuits.

If you can duplicate this event with any regularity, unplug the coolant sensor and run the car while monitoring that sensor circuit to see if the glitch is still there. If so you know the problem is the PCM. Only down side is the engine will run very rich with the coolant sensor unplugged. That monitored value in TunerPro should read -40F or so unplugged.

[Karms20]

Well there is only 3 things it could be..The sensor, the wiring, or the PCM. You have ruled out the coolant sensor. The harness can be the next easiest ( well cheapest anyway ) thing to verify.
Odd that it jumps to a high temperature, indicating a short to ground somewhere in that circuit.

The vehicle has the original spark plug wires (21-yrs old, 80K miles). Is it possible the spark plug wires now have deteriorated/leaking insulation allowing sparks to intermittently leak through the insulation to the engine coolant temperature sensor wiring creating an induced voltage?

This intermittent increased voltage would then be "seen" as an intermittent increased engine coolant temperature by the ECM.

[Six_Shooter]

While I don't believe that is what is happening, I would definitely replace those 21 year old spark plug wires.