Just how separate is the RX supply? The P92 has a BEC to supply the RX, and, through the RX, the servo and the input side of the P94.
Servos work by comparing the position that they think they are at with what their incoming instruction tells them it should be, and driving the motor to eliminate the difference. The reference is derived from the supply voltage of the servo. If that fluctuates due to changing load, the servo tries to keep pace, often changing the load. The radio could also have it reference voltage messed up by changing supply voltage.
The P92 and P94, between them, should have a good solid ground line connection, with power only going via the heavy wiring, but that is worth re-checking.
Less likely with modern radios is motor generated interference, but a frequent offender on AM systems. This usually had the most effect on Channel 1, almost invariably the rudder channel.