No video on boot

All, the below machine booted earlier but the display kept cutting in and out and the screen appeared to be auto adjusting constantly. After about 30 seconds, the machine restarted but there is no video.
I have power to HDDs, disk drives, GPU, RAM and I can "hear" Windows booting normally I.e. by the noise of the fans and drives.
I've tried clearing the CMOS, removing the battery and trying to get rid of all residual power in the machine but still I have nothing.
The machine is not overlocked and doesn't run anything taxing.
I get power to the keyboard but even if I try to start in safe mode I don't get a video signal.

Gigabyte GA P55 UD3R
ATI Radeon 4770 512MB
Intel I5 processor (can't remember exact spec)
Freezer 7 Pro
Be Quiet Dark Power Pro 650W
2 x Seagate 1TB HDs (RAID config)
2 x 4GB Corsair Dominator DDR3 RAM

From what I've already found online, looks like I need to test another mobo? Or should I try the processor first?

Any help/guidance would be greatly appreciated...

The motherboard is probably shot