Temp Gauge swings toward high-side and back to norm with 3 sec

Wild needle swings are a symptom of failing instrument voltage regulator. I've seen this quite a few times. In most cases this temp gauge swing doesn't happen at cold start and warm up. It happens only after water is hot, and ignition switch is cycled off then on again. Anyway...
Eventually the limiter contact points will stick permanently, and something will die. Maybe the one gauge?, or all of the gauges?, or hopefully only the limiter itself.