Thanks for the reply.
Yeah I plugged it all out and cleared the CMOS.
Ive read that pretty much all this does is reset the BIOS, so if its a corrupt BIOS file or a setting that's conflicting it basically sorts that out. (as well as reset things like the date and time etc)
The fact that this seems to sort it out at least for a while leads me to think its a BIOS problem.
As i said to CloudForever, its going OK nearly 5 days now since but if it goes again ill try your methods too and will let you know what happens.