Anurag,
This may indicate that there is some slop in your linkages or hinges. I have had this happen with digitals also and this was the cause. The servo is trying to take up slop but overshoots due to the slop, and then reverses to do the same again, and is trapped in this loop. I guess the servo is sensing a varible load. I my case there was some slight slop in the hinges, and when this was removed, the problem was gone.
Regards,

John