Many thanks for all suggestions.
The problem was indeed with the PSU which I have now changed and everything seems to be working just fine.......except for one thing.
CD writer (Plextor 48/24/48A with 1.07 firmware) point blank refuses to function in DMA Mode.
I've tried everything suggested in the FAQ's, scoured the boards for ages and still cannot get the drive to function in DMA.
It always defaults to PIO and fuctions well but very slowly (4-5x) using Plextools.
Using EAC the speed in secure mode reduces speed to 2-3x.
Any further suggestions? (or is this particular Plex about to be discarded!)