Are you sure this is a 5955 servo verses a 5995 as this would make sense in this circumstance.
Can you try another servo on this channel to validate the servo is the culprit?
Yes, they are 5955s.
I connected my rudder servo to the elevator channel and get the same result. The servo for the other elevator half, which is running off a separate extension on an AUX channel (mixed with the elevator channel), also stops moving like the servo on the elevator channel when on high rates. It doesn't seem like a servo problem since all 3 do the same thing.(?)
Dan