RabbitB, before your OH takes the scanner apart, make sure that the connectors at each end are good and tight. You can get some funny effects with scanners if the ground connection isn't good at each end.
Do you know what sort of scanner it is ? Does it have a thin fluorescent tube doing the scanning, or is it a row of LED's ? Does it connect via a USB plug, or is it one of the older ones that connects to a printer port ?
If it's a tube, see if it's flickering. Also, listen for any groans and squeaks coming from the mechanism when it's moving. Sometimes the rails that the scanner head runs on need a little light oil.