We have had this issue too, 3 SA8300HD boxes in a rack, ch +/- would go up 1 then skip 2 or 3 channels every few presses, moving around the guide same thing. This only happened with an XP-8.
We ended up adding an RP6 just for the cable boxes.
Then I was given a possible solution, I haven't tried it as I had already moved way too many buttons over to the RP6.
Create a system macro in the XP-8 for CH +/- and other buttons that may be having problems.
If you try this let us know if it works, I think it should.
Cheers,