WHERE are you doing this? Basement? Garage? Shed?
Point is when testing radio installs in a closed area like the above, reflections off the surrounding walls, heating/AC ducts, etc can often cause one or more servos to jitter. The transmitter too being only a couple of feet away or less can simply swamp the receiver and cause one or more servos to jitter. Especially the less expensive servos and when batteries are fresh and fully charged.
Take the whole thing outside, away from buildings and try it there. If it continues to happen then move the Tx at least 10 feet away from the Rx. If it keeps up then I'd suggest unplugging the cheap servos in the model and plug in some regular servos of a brand name and keep the ESC plugged in as well. If they behave normally then I'd say those cheap servos are to blame and should be changed. If not then unplug the ESC and see if things settle down. If they do then plug the cheap ones back in without the ESC and see what happens.