Depending on the on your actual machine, it is quite possible that it is just the "Counter Balance" for the Z Axis. Some machines have an Air Cylinder with it's own Air Pressure Regulator so double check the Air Pressure Setting. Too high and it could happen when you travel down, too low could happen when you travel up, but it could be the oppisite, again depending on your machine. Other machines have an actual "Counter Balance" weight, make sure it is still attached or for that matter make sure that how it is connected is well lubricated.
One thin to try just to simplifiy your trouble shooting is, in your program prior to stopping your Spindle, do an "G4K500", this will add a delay prior to stopping the spindle.
That Drive Error is usually because the Drive is trying to the oppisite of what it just was to it needs to "Disapate" the excess energy, which is typically what the metal box on the out side of the control is, Dynamic Braking Modual. Unused energy is attempted to be fed back on to the "Bus" so the other drives can utilize it, but if it doesn't get used generally it is "Dumped" onto the resistors in the Dynamic Braking Modual. If it can't do this than the drive Errors out. So assuming the Counter Balance isn't the issue, make sure that the Dynamic Braking Modual is free and clear of any metal shavings, which can short out the resistors not allowing the energy to be disapated on them.
Hope this helps. (sorry for the spelling, in a hurry)
Motion Master 5 Axis Router.......Fagor 8055/C.......Surfcam Velocity 4.0 B189