Quote Originally Posted by Chike View Post
When I'll face such a dilemma I'll have to think about it.
Maybe you want 650W that will suite future possible upgrades, like GPU or even motherboard/PSU.
I picked up a cheap 30 500W PSU from Amazon to test and (fingers crossed) it appears to have fixed all the issues. Shut downs fine now and BIOS setting are preserved even when unplugged. I'm guessing the PSU was always supplying current for some reason which was spinning the fans, e.t.c., and causing the BIOS to flag an error which led to it restoring from the backup BIOS. Just a guess though. Thanks for all your help :)