Symptoms of blocked integral bypass on combi.

[Sharp Point]

Evening chaps, what would the classic symptoms be if the bypass is blocked by particles of scale or muck?

offending boiler is a Vokera compact 25he in a rental property. Was called out to hot water going hot cold hot cold etc.
If I had a Pound for every boiler I've been too with this!! I got the old gallon tub of 'brick acid' and cleaned out the plate heat exchanger and refitted......but the boiler is still cycling after a minute of hot water operation. (The hot water is amazing now, going by what the tenant said!)
Now I'm thinking the pump maybe tired or impeller full of crud( to late to check)would a blocked bypass play a part?

 

[lame plumber]

doubt it

 

[GrahamM]

I assume you defo ruled out the age old favorites of dodgy thermistors on these!