Just in case anyone still cares about this problem, it turned out to be a conflict with the SCSI cards and the scanners (believe it or not). Once a certain volume of data had passed through the SCSI card it just seemed to close itself down until the PC was reset...

We had the latest SCSI drivers and everything, but that seems to be the cause of the problem. Fortunately the rate at which they are scanning means they switch the PC off at the end of the shift long before the SCSI card packs up.

I'd just like to thank everyone for the help they gave with this.

Paul Anderson