a wierd gauge issue on 1994

[Marv D]

not sure wht the heck... Have a A-pillar guage set, sensor is in the intake beside the thermostat housing. It syats ROCK SOLID at 170°
BUT the dash temp gauge started making slow sweeps up to 250°+, sets the 'check gauges" lamp, then drops back to 170ish,

I replaced the factory temp sensor in the drivers side head (a 1 wire sensor ) ,,, does exactly the same thing. Unplug the sensor and gauge drops to 0,

With the pod gauge staying consistant,, I brought my Fluke certified temp gun,, and the thermostat housing is staying at 170 even if the dash gauge says 250, And I know the smell of a HOT motor (living in the Arizona desert) It's just not hot!

The sweep from 170 to 250 will take 30 seconds or so. I tried grounding the sensor wire and it flashes to PEGGED the dash gauge.

All of that to say I have to believe the motor is NOT getting hot unless there is some weird steam bubble happening at that sensor. But I'm not loosing coolant, no coolant in the oil, radiator is full (no bubbles)

is this a common issue?
Anyone have ot know of a gauge problem that may cause this. I have a guy that rebuilds 04-06 GM gauge pods when the resistors fail and they dont 'resert' But I can't get ahold of him.

Looking for insight