I use 7955s on both elevator halves, ailerons and rudder using a JR9 channel receiver with no problem. I'm fairly certain I've used several different Hitec servos (including 7955s) off of my Spektrum 7 channel reciever in the past with no problems either. My first thought would be perhaps either one of the 7955s is bad/has a bad connection?? Have you tried individually swapping the two servos for the JRs to see if the problem only shows up when you put a particular servo in the circuit?? How about battery levels? What type of battery are you using and is it charged fully? You didn't specify your exact wiring configuration so don't know if there are extensions involved that might be introducing issues as well? Perhaps a bad connection that you "fixed" when you plugged in the JR servos? Look at the connectors on the 7955s and make sure a pin is not backing out... that kind of thing.

The signal issue the previous post refers to I have never heard of being a problem OTHER than with Futaba who has posted a bulletin to that effect so it shouldn't be an issue here. That's a particularly Futaba problem from everything I've heard.