Sanwa Exzes X problem

[Npdp72]

I have recently bought a used Sanwa Exzes X with RX-451 receiver and I find I get jittering / twitching of the steering servo at the neutral position.
I am using a Savox 1258 servo (which is brand new) and have tried switching over to my spare Savox 1258 and I get the same thing.

Could this be a problem with the radio gear?

Can anyone shed any light on why this is happening?

I'm using a LRP SXX V1 ESC if that makes any difference.
I know of someone else at my club that uses the same radio gear with the same servo and he doesn't have this problem.

Thanks in advance for any help.