vaillant woes?

hi all ,in need of a bit of help for a sick vaillant 828 turbo plus
fitted about 3 years ago,a brief history>>>boiler fitted but system was
not flushed first(i know) hence never worked properly from word go had
vaillant engineer down diagnosed secondary heat exchanger blocked (full
of black bits)cleaned it got boiler working and recomended a power
flush.system power flushed(plenty of black dirty water)boiler sort of
working better but gradually got worse so brought new heat exchanger
and still the same next fitted diverter valve still no joy whats always
happened is the boiler takes ages to cut in when there is a demand for
hot water, heating works fine, sometimes hot water demand works fine
sometimes 20 mins or so for it to cut in
if any one can steer me in the right direction it would be much
appreceated.