Monitor Problem ?
 

Every now and again my monitor will go blank then a message will popup saying 0khz - 20khz out of sync or something similar.

Normally restarting X solves it but the last time it happened the display was mangled and a cold reset was required.

Anyone shed any light on this ?

The first thing I'd check is to verify that the video cable is connected securely to the video card. Loose connections can produce exactly the symptom you describe. That may not be the issue, but it's definitely something to check. -- J.W.

and also check out your XFConfig and see what the refresh rates are set to and all that.

Thanks I'll try that.