Thanks for the excellent advice Xanthe.

1)  We are in winter here - the room is very cool (minus the PC heat).

2)  No cards are not overclocked.

3)  Wow that review says it all.  Seems this PSU is indeed probably the cause.

Question,  it ran fine for just over two months (on GPU mining) before the problem started - is there a possible explanation for that?  Possibly wearing down already?