Rocking and wobble are the same thing. SO you saying it rocked but did not wobble is a bit confusing. It either does or it doesn't. Though if it were slightly misaligned, it would seem to rock when spun.
The Flex 940 and Combo 910 are practically the same machine, the only difference are the base clamps. The Flex 865s is really a table top machine on a stand. If the Flex 865s has the same center mount of the Flex 740 it's totally different from the Combo 910/Flex 940. It's kind of backwards in design.
I think it's the disc brake that causes some of the wobble. The screw brake on the Plus 6500 is the same screw brake on the Flex 920. Much more simple in design.
Machines: Gamma 6004 2-point w/ Wise 2086 & Babolat Sensor Dual