Update: The nVidia panel was showing the screen refresh at 50hz , which is OK for UK, however in Windows screen properties the refresh was shown as 59Hz. They are both set at 50Hz now. Whether this has solved it or not I am not sure, but I now have full HD playing using VLC and I am monitoring the GPU with nVidia Inspector, and the GPU load for full HD is being reported as 21%.
I am going to try MP now....
Thanks Xetic for your input, invaluable advice and I am grateful.
EDIT: Crashed again - noticed that PCIe voltage was set to manual in the BIOS, so changed that to auto, also changed codec in MP from MS-DVD/TV (which has served me well for 3 years) and now running LAV with GPU load being reported as between 35% - 54%. I'll run LAV for a bit to see if it is any more stable than the Win7 codec. Oh, dear just got BSOD... then a reboot - well, it makes a change. I'll be able to post logs up now