Hi,
I've not yet had exact this problem, but I think I can explain what's happening.
I don't know how many knowledge you have regarding the steering of servos. The position is defined by HIGH signals in a defined length in milliseconds. In general you have 1100ms for complete left, 1900ms for complete right. All signals in between are the positions in between.
If a servo gets a command, it calculates the actual position from the potentiometer. Depending on that value the servo drives the arm left or right until the value from the potentiometer fits to the requested value.
Now your problem. You added the capacitors and by doing this it you defined a situation that will always be calculated to the same value, e. g. 1513ms.
As long as the center value of the receiver fits to this fix value the servo will stay in that position. If the center position differs slightly, e. g. 1528 the servo will identify this as a difference that needs a small correction. The arm will start to move. Because of the fix capacitors the requested value will never be reached and the arm will move continously.
A solution is a bit complicated. First you must find the ms value that keeps the servo quiet. Than you can program e. g. a ATTiny85. It must read the value from the receiver and as long as the difference between the receiver value and the found out value is smaller than a defined threshold, the chip sends out the fix servo value. If the difference is higher than the threshold the receiver value is passed to the servo.
That's it...
Even if I do not have an ad hoc solution I hope I could help to find out the reason. This might help finding a solution.