Servo or Encoder problem?

I'm in the process of upgrading from steppers to servos and have
come across a slight problem. I've finished my control box
containing the power supply, Geckos and all the cables and tried it
(on the bench) and everything seemed to work fine. After some more
testing to today I noticed that one servo performs erraticly at high
speeds. Two of them run fine up to nearly 4000 RPM but one seems to
shake and cause the drive to fault after about 2000 rpm. This
problem is in the servo as I tried it on another drive/input. I'm
just wondering if its the encoder or the motor? Everything works
fine at lower speeds. What could be causing this? Thanks in advance.

Mark