Here goes !......BMC 30 problem.

Basically the Z axis will not settle. Constantly moving about 0.1 to 0.15.....
When i callibrate the machine, Z is not zero but 0.700
Also, if i press either Manual,Input,Auxillary or Review, the Z axis bounces and usually causes a Z Axis following error.
If left sitting for a while , the axis rumbles and bounces and invariably errors out.
So do i need a new encoder...or drive, or can i tweek anything to try correct it ?
It has somehow corrected itself before, and run perfectly for weeks, then out of the blue started this again.
Any help woulkd be much appreciated