Servos jiggling is never a good thing.
I had that problem recently with a Versa Wing I’d made, and the reason for it was because I’d bought some really cheap servos (I think it was like 10 for $25-$30?) and they had power draw issues. I thought it was a receiver, so I swapped out to a Spektrum brand receiver, and still the same issues. Plugged in a known good servo in its place, and the jittering went away. Swapped out the receiver back to the original, and still no problems.
This is a direct result of me trying to buy the cheapest materials - quality control was not there on these things. A servo tester with four AA would push the servo, but the ESC would not. And since swapping out the servo with the same ESC and receiver fixed the problem, I was able to determine it was an issue with the servos, not the receiver or the ESC.
In your instance, I would start by making sure the transmitter battery is charged, and the battery connected to the receiver is charged. Check for loose/damaged wires or connections. If you still have the same issues, start by swapping a component out, one at a time. This doesn’t mean you need to cut out the existing servos yet - start by just unplugging one and connect a different one on the bench, and see if it freaks out like it did before. Try and do the same thing with your ESC and receiver, if possible - replace it with another known good piece of equipment to see if the problem goes away.
