Repeater Regions are stored separately in different XML files per character in the IS Scripts folder, not in your ISBoxer config itself.

I think you may have misunderstood what I said in my previous reply, and I don't know how your config is set up to tell you what's wrong exactly. I'll explain with more detail.

The window you are physically moving your mouse in at the time is the only one that needs a Repeater Region, and if you're using it for healing, you should select the healer (or the healer's ATG) for the Target after checking "Block local click", and then click Add. Note that you can't edit these options after your region is already made; if you are creating a region and then trying to edit it later, this would be one potential reason for your confusion.

You do not need to click Sync for the region to work, it's just going to work already from the window you made it in. The reason to click Sync is if you are going to be physically clicking in a different window, and you need THAT window to automatically send the mouse from that region to the healer also. You can remove the region from any given windows after Syncing: Go to that window, hit the in-game GUI toggle hotkey (ctrl+shift+alt+g by default from the wizard) and click the X on the region. Note that while the GUI is toggled on, the title bar on the region shows the Target for the region. If the target is all or all other, then this is probably the problem.

You do not need to "disable the mouse on the slave machine" -- mouse repeat is off by default, you're configuring it to be enabled at the time (this is the problem) -- you just need to make sure they aren't fighting over mouse repeat. There's a few different ways that might be happening. First, if your repeater region is present in the foreground/active window on both PCs, and the Target of those regions includes the foreground/active window on the opposite PC, they are going to fight. If you have Mouse Repeat enabled in the foreground/active window on both PCs, they are going to fight. If you have Mouse Repeat enabled in the foreground/active window on the second PC, and the first PC has a Repeater Region that sends to that window, they are going to fight.

If you configured hotkeys in your Repeater Profile, or you changed the "startup defaults", these would be potential reasons why the problem might happen.

I gather one should be able to send a mouse repeater region to multiple computers without jittering issues? as in a 5x shaman healing via grid and clique?
If you're trying to heal with all of your guys via a Repeater Region on multiple PCs, you will need to pick a PC to control from, because if the region is on all and sending to all, they're all going to be fighting.

The entire issue here is that ISBoxer does not have the ability to determine which of the mice is doing the actual movement, so only one PC at a time should be doing the broadcasting or it's going to be jittery. Joe Multiboxer does have this ability and will have such no problems with mouse broadcasting on multiple computers.