Ok - an update.
Yesterday morning I replaced the battery for the CMOS, held down the pwr button and clear CMOS button for a good 2-3mins, and then tried to power up the system (with only the CPU, 1 stick of Ram, GPU and PSU connected - sitting in the case).
The mobo posted, but went into a really weird BIOS loop, where it wouldn't get to the disk boot failure, just the initial post screen then restart. The mobo had recognised however it was having posting issues (gee, thanks
)
So I turned the power off, and tried to get it to post again, nothing. So I took the mobo (with the CPU, RAM still attached), grabbed my other ram sticks and PSU, and took the lot back to where I purchased it from for testing.
They put their own stick of RAM on, used my CPU, their own PSU, their own GPU, and ran it outside the case, and it posted first time, everytime. So I got them to put my RAM, GPU and use my PSU, posted first time, everytime (I was thinking WTF). So I put all the sticks of RAM on, posted everytime.
Bought the whole lot home, installing one piece at a time (back in the case), posting everytime. Long and behold, the whole system is back together now and didn't fault along the way of the install (i was testing it every new component that I added back to make sure it would still post, and then rebooted it about 4 or 5 times each test, just to be sure). The OS was installed back on, and the computer was running fine last night.
Go to start it up this morning, and I get a weird post beep - 1 long then 2 short - which according to the Gigabyte manual is Monitor of GPU error. So I try to restart it, same thing. I hit the power button on the case, and try and reboot, posts no problem, boots up the OS, and has been running flawlessly since.
Does this still sound like a mobo issue, or could the GPU legitimately be faulty? (I'm struggling to see how it could be the GPU, as it only does it very intermittently).