I have a futaba 4vf transmitter w/the original futaba fpr127df receiver purchased in 2000. It was functioning normal but later in the flight the plane started to get finicky. While doing a range check w/ the antenna down it won't make it past 10 paces before all the servos get jittery, (4.8) battery tested 5 volts w/ a load. I swapped a different receiver and transmitter in the plane, it performed flawless. I began trying the radio out of the plane w/ different servos and batt packs indicating 5+ v w/a load and it still jitters past 10 paces. Is this radio system basically junk, or is there something else I can do.

Sounds like you have a bad receiver. You can send it in to be checked out, or just buy a new one.

It could be any number of things wrong with it. I would either send it in for repairs or toss it in the trash and replace it. It is not worth loosing a plane over. Good Luck, Dave