Peter,
I agree with Paul. The DSIII should be no problem. Are you using a guide scope or off-axis guider. I've used both with a GSO RC and both work fine.
The focuser supplied is not up to imaging standards but a feather-touch with motorised focuser is the duck's guts with these.
Contact Ron at Sirius Optics (
http://www.sirius-optics.com.au/ email to soptics@bigpond.net.au. They're at Underwood, Qld, Australia 4119
Phone: 61 7 3423 2355, Fax: 61 7 3423 2366) I know he has a second-hand RC 8" with a feather-touch and a JMI motofocus on it for sale.
Peter