Hi Tim,
Thanks for the info. This amp has been modified I guess at the
factory as it was made mid 1997. The wires were soldered to the board
& covered with hot melt glue which I think is a bad thing. I have
removed both input screened cables & replaced as a precaution.
Sadly it has made no difference to the problem.
I am checking the ground connections but I still suspect the design of
the PSU is inherently unstable. I can induce the fault by switching on
my heat gun & holding over the PSU board. I was trying to see if the
problem is heat related but it seems not.

Clifto - I don't understand your comment. I have replaced the screened
cables as described above but are you suggesting the power leads need
screening too?

Best regards,
Greg.