Worcester Greenstar ERP 30i HW water problem

[Coopyrob467]

I have been out to a 2 year old Worcester Greenstar 30i with an intermittent problem with the hot water going hot then cold (clap cold like the cold tap is on). I have looked at the pattern, the water temp is set at 50 degrees the boiler fires up when the tap is turned on the temp climbs as it should up to 50 degrees then slowly goes past that up to about 58/59 then the boiler shuts down for about 15 seconds while the temp drops then it fires up again, now iv noticed that after a few times of doing this it actually stayed at between 50-53 and the boiler stayed on and the water stayed warm.. no problems, but when the water was turned off and then put back on again the boiler did the same again but this time it didn’t settle but I could hear it clicking but it seemed irregular and not as it should be. I removed the plate heat exchanger yesterday as that was the obvious cause and it was clean as a whistle, I did give it a flush with some one shot just in case but the problem is still there, can anyone help with some other possible causes, maybe a temp sensor? Thank you for any help.