I'd say maybe take the cover off the head and make sure all the cables are tight. If that doesn't fix it, call Summa tech support, they are the best in the business if you've never dealt with them before.
Otherwise, software maybe? Dunno if if the software you're cutting from can turn it on or off, I assume it can't, the OPOS sensor on our S160 is always on, not sure if it's different on the D series or not.