Vaillant Turbomax 242e water running hot then cold then hot?

I have a Vaillant TurboMax 242e. The hot water runs hot then cold then hot, with the burner turning on then, presumably overheating and turning off, then on again repeatedly. The temperature guage reads just under 90degC which the water from the tap isn't reaching. I'm assuming the diverter valve isn't opening properly to allow a decent flow of water into the heat exchanger. Any thoughts on the symptoms and the solution.

Cheers