Random characters, then death

[theprophecy]

I just got an IWill XP333-R. It's replacing a board in a perfectly fine system. On boot, within about 15 seconds lines and random characters start to appear on the screen. Within 30 seconds the screen is unintelligible, and after that I shut it down, cos, well, I don't know what's causing it and I don't want to break something.

So, what's the cause?