I am familiar with at least two other KingCats with the same setup (except using 955 RX) that have had the exact same problem. For me the solution was to run individual extensions directly from each servo to the RX, using a clamp-on style JR choke on each.

I think the problem is noise related and caused by running two servos on very long extensions through a "Y" and a single choke (on the short leg of the Y), but that's just my guess.

Barry