Seem to have fixed the issue. After removing the RAM one by one and still failing to complete memory diagnostic, I removed GPU, uninstalled all drivers, put the GPU back and installed new drivers and It now completes memory diagnostics without showing any errors and SEEMS to be playing games/videos ok...

I have no idea what caused it, but that seems to have fixed it for now. All part of the fun of windows i guess...

Thanks to all who posted

TK