Ubuntu – VNC Performance

When looking at the two machines having problems, do they have the same video card? If so, is their video card different than others that are working fine -- or different drivers?

Do you have the mirror driver installed? If so, have you tried uninstalling it (or vice-versa)?

I'd suggest a better idea would be to use the built in functionality in ssh to tunnel/port forward some local ports on through your ssh client to the destination host, and use rdp on the windows boxes, and vnc on mac.

For the linux host, if you want graphical access to it, i'd suggest no machine (NX).