Heatline Monza 28A?

Hello I have a Heatline monza 28A combi boiler which I have just started to have problems with.

The heating side of the boiler is working perfectly but the hot water is a bit strange and intermittent , when I turn on the bath taps the boiler fires up and lovely hot water flows for a while and then suddenly it goes cold as in the same at the cold tap cold not luke warm and then a few seconds later it gets hot again and so on. I went and stood with the boiler to see what happened when it did this and the boiler keeps running but when you feel the temperature of the pipes when the DHW goes cold the CH flow gets red hot as if something is switching the diver valve. When the water is running normally the CH flow is cold so not as if the DV is jammed it’s actually switching between DHW and CH for a few seconds.

Any ideas iv ordered a new plate heat exchanger, Diverter Valve repair kit and DV motor to see if they help but im not convinced, could it be a temperature or pressure sensor that’s opening the DV and dumping the hot water down the CH Flow ?

Thanks Russ