vaillant eco tec plus 831 intermittent hot water problems

Hi All, Just to say I am a end user not plumber.
Have a problem with the hot water. It intermitantly runs tepid. About 3 months ago I had the Diverter valve changed as I had no hot water at all.
Since then I have had tepid water intermittently. I happens mostley first thing in the morning. The boiler starts on a high number aprox 75 but then gradually drops downd to aprox 50 and stays there. During this process the boiler makes a shuddering sound. I can restart the boiler and it then it works fine. Ther are no error codes I can see. The heating is fine. Bar is 1.5.
I have had British Gas around twice. First time couldnt locate problem. It was a cold morning and sugested water pipe to drain could be frozen. I have since put new insulation foam on it. It then kept happening again in the warmer weather.
They came back again last week, still could not find a problem so fitted new circuit board. It has now happend again 3 time since this was fitted.
I have taken video of the boiler during the fault process inc the shuddering noise.
Can anyone make a suggestion what it might be ?
I have a contract with British Gas so would like to use them.
Thanks

Sounds like you need to contact the complaints department within British Gas. If it’s intermittent and only happens once then I’d be looking towards the diverter again, and anywhere in that vicinity.

Thanks for your advice. The problem is happening about 2 or 3 times a week They are I coming in on Monday. Just trying to shed some light. Thanks