spindle counterweights

Hello,

I have a CNC machine that I have retrofitted to a more modern
control.
It is a bed mill with a 42"x20" bed and a 5 HP spindle motor.
It was previously only capable of 2 axis simultaneous motion. It
had a dec pdp control. We have are now running a windows
based control with a galil servo card.

Anyway, there is a counterbalance on the spindle that consists of
a chain that goes up and over the top of the machine (via some
pulleys) to some weights that hang on the back of the machine.
We experience some problems when there are rapid
accelerations away from the bed and it also tends to overshoot
when moving toward the bed. It will sometimes gouge when it is
move down and has to stop rapidly.

I have been planning on writing some software to preprocess
the G-Code and look for rapid accelerations in the Z and add
acceleration and deceleration for those lines but I was
wondering how other machines tend to handle the
counterbalance issue since I think some of our problems are
due to that. You can hear the counterbalance bouncing around a
little under rapid acceleration.

Does anybody have any ideas?

Thanks,
-Jeremy