CHT Issues

Make sure the connector at the probe is snug and not loose. if it's loose you can squeeze it with a pair of pliers. It could be a loose connection somewhere else on the wire into the gauge.

Have you definitely got coolant in the reservoir on top of the crankcase? Do the coolant hoses all feel hot?

You could swap the CHT & oil temp sensor connections and see if the high temp moves to the oil temp gauge. If it does then you have a sensor fault or genuinely high temperatures. If the high temp stays with the CHT then it is a gauge fault. If it is the former, then you'll have to swap the sensors (they are the same) to see where the fault ends up.