My boiler is slowly torturing me...

A while back it started making a nasty clonk when going
from central-heating mode to hot-water mode. After a bit
it started to intermitantly fail to give hot-water on
demand.

A new diverter valve was fitted (the second in 2 years,
and the boiler is less than 4 years old).

All seemed well for a few days, but now it's clonking
with avengance once more!

Any ideas? I've run out of them!

G.