The outputs of many receivers can only drive a 5K ohm or lighter load due to their CMOS driven servo signal. I have a feeling that the offending servos are conflicting with this limitation.

If you have a good ohmmeter then unplug the servo and measure the resistance from its ground to the control signal (across black and yellow). Reverse the meter leads and record readings both ways.

Standard servos are usually 20K to 50K ohms. What are yours?