Have you checked the little slide on spade connector on the temperature sensor, sited on the front LHS of the engine next to the thermostat housing, may be working loose a bit and varying its contact resistance while umping along the roads. I would also be looking at the voltage stabalisation across the +ve and -ve feeding the temp gauge as well as wondering about the temp gauge itself

No, I'm serious, look in your header tank tank, then give bike a :devil::devil::devil::devil:ing good kicking.