I'm not the manuf., but I'll tell you my experiences....

I've used both PCM and PPM and the only difference I see on my planes is the perceived noise immunity in PCM (the bad signal packets are discarded, thus shutting down the channel while the signal is interfered with when using PCM, rather than the servos jerking around such as when using PPM), and the failsafe.

I have not noticed any problem with jittery servos, etc. unless like you, I am in a noisy environment.

Since you do have a shop with excessive RFI, you may have hit on the problem. Try it at a different location and see if it gets better.

Good luck!

Sean