Marantz P400 cuts out at low volume

Hi,
also not tubes (although i do dabble in the valve DIY) but perhaps
worthy still. please help

managed to pick this great amp up for free (with full system) but it
has started to play up.

one and sometimes both channels fuz and cut out itermitantly. It
seems to be worse at low volume settings and will usually cut in
during peaks such as kick drum beats or when the vocals start. my
best bet is perhaps a capacitor in the power amp stage has failed but
which one?

i have checked for dry joints as best i can and there also does not
seem to be any speaker protection relays that may be playing up.

any help?

.