Do they chatter or do they "sing"? If they chatter, the linkages may be fighting each other if 2 of them are connected to the same flight surface.

It could be that your battery voltage is down too. Check your extension cables for tight, clean connections. Better yet, hard wire longer leads to the servos for minimum voltage drop.

I don't think "RF Filters" (whatever they are, maybe chokes???) will fix this problem.

Most digitals will sing, or produce a rather high pitched whine when under a slight load. That's normal. Un-Nerving, but normal...