Ravenheat HE 85T intermittent hot water in shower and bath.

We've got a Ravenheat HE85T (I know, its a cack boiler) and it's recently started palying up with the hot water.
When we use the shower or bath, the water is hot for a while, then goes cold, then heats up again, as if the boiler can't keep up. The water from the taps in the bathroom sink and in the kitchen are fine though. They get hot and stay hot.
The boiler fires up as it should when the shower or bath is run, and hasn't cut out (as far as I know!)
Is it likely to be a flow rate problem? The bath taps stay hot if I turn it right down to a trickle, but the shower won't go to a flow rate low enough where the boiler still keeps running. Even on it's lowest flow the water runs cold before heating up again.

So far the fan mod has packed up (known problem, fixed by Ravenheat under guarentee) and the diverter valve was replaced last year.

I don't know a huge amount about boilers, most of what I know is from reading other posts on here, but I'm prepared to try a few fixes myself.

Thanks in advance for any help!

Measure the minimum flow rate at which the boiler will fire up!

Not my favourite boiler following wasting over an hour with one yesterday and needing to return with a spare part soon.

Not really the boiler's fault but the owner chooses to have a leaking gutter to drop water over the flue.

Tony

Minimum at the kitchen sink was a shade over 3litre a minute. That doesn't sound like much to me, but what do I know!

So, any ideas?
I've got a quote for a powerflush at £530, but is this likely to sort it out at all?
It probably needs doing anyway, but I'd like to get the boiler working properly first.

Thats an expensive power flush!

Should be about £350 from an independent.

But nothing you state makes me think that its power flush related.

But as always it probably will need a competent engineer to diagnose the fault.

Tony

Dhw going hot/cold/hot is often a sign of a badly corroded/soiled system. If that is the problem here, a powerflush can solve it, but only if done correctly.
We don't do price comparisons here, but your quote does sound a bit hefty. Make sure you get a written guarantee that it will solve the problem.