Just wondering if maybe it was shorting up against the back panel, when the board touches the steel it bridges two points and causes abnormal operation. It happens sometimes. Could also be the screws into the chassis as well considering they press against the motherboard but this is a lot less likely as most good boards will have special contact points around each screw to assist with proper earthing and the like.
Could be anything, but good to hear you got it working anyway. Not bad a spec either, 850mhz is still a very capable and sufficient system.