How constant is the reading on the Stack dash? Mine seems to jump about all the time within about +/- 5 deg C
Check your earth from the battery to the chassis, because if it is at all ‘iffy’ you can get those symptoms.
Let us know if that cures it, for posterity
It can vary by that much naturally, but if it’s happening instantly, it’s most likely a dodgy temperature sender. They’re a crumby part that don’t last long. Thankfully they’re cheap from the usual sources.
The fact it is a digital readout means you notice very small changes that wouldn’t register on a dial gauge.
Or, as Mike says, a dodgy earth. The way to test is by sitting in the car and changing the electrical load, e.g. by switching the lights or the cabin fan on and off. If that makes the temp jump up and down instantly then the engine (or battery) earth staps need cleaning up and refitting.