I had similar problems originally. Using ip adresses instead of host names helped some. To avoid that I loose the mouse, I have set Multibox's screen setup to only the primary screen. That robs me of the option of using the primary computers mouse on the secondaries, but it also completely removes the problem of lag and disconnections when the mouse if moved to another computer's screen.

Keep in mind that Micah put together Multibox primarily for himself and shared it here free of charge. It is not bound to be as polished as maintained and charged software.

While it has other limitations, I understand that keyclone is a valid option for multiple setups, and while you do need to pay for it one time, it is maintained and supported.