Right then, being more realistic - I've backed my 24/7 overclock down to 4.8 for now as temps (and volts) are easily controllable at that speed.

So I rebuild my system last night (new case) - reset CMOS (fully by pulling battery), and booted at stock settings / speed. Ran a quick IBT and shutdown.
When i booted up I then got stuck! - It refused to boot. Tried this four times in a row - eventually it booted. Tested it again by shutting down and starting up and worked fine this time.

Looks like my booting issues aren't just overclock related!! grrrrrr..Can't be my memory as I've had two different sets in here - one Mushkin the other Corsair. Can't be overclocking as I get the same. Can't be the CPU as I got the same with my 2600k and now my 2700k. Only leaves one thing - the mainboard!

Come on ASRock - give us a new BIOS before I return this thing!