[ricm]

I've got 645's on all my trucks for steering only....they are way too high spec for the shifting or the coupler, so I use standard Futaba S3001 for those other functions. The 645's do drawing lots of current, if you find that unplugging one of them solves the problem for the other servo, the I suggest down-grading your shift servo to something lower spec which will still be perfectly adequate for the job.

it -might- be that the brushes on the centering pots are worn, this does create the same 'jittering' effect. But I've only ever had this problem on cheap 'reconditioned' servos from China - never on new OEM brands like Hitec. It is possible but extremely unlikely that both servos are suffering from the same problem (one, maybe, but both?)....unless they've had a hard life in another model at some point...