Those gauges work or they don't, period.

It could be low water or bad pressure cap allowing a bubble to form in the waterjacket. This will have steam in it and as the water level surges the probe is alternately wetted and "dried" causing severe fluctuation, however...

I suspect a hyper-gauge. Try blocking off half of your radiator first and see if that doesn't stabilize things. My Dodge with a turbo Cummins is renowned for this little issue. Lots of "fixes" but the best I've found is blocking part of the radiator to keep a slug of cold water from closing the thermostat which then opens and gets a slug of cold water and closes and etc. ad naseum. Blocking off part of the radiator reduces the temp differential of the slug of cold water when the thermostat opens (so it doesn't spastically close again).

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Mike!