Make sure that the "Home slave with master" is not checked in the Mach General Config.
The Galil plugin puts the master/slave axis in gantry mode. But if Mach sends movement commands to the slave axis, then the movement of the slave axis is doubled!
In the next release, I will handle this case in the code so that no matter how the General Config is setup, it will do the right thing.
But... as Kenny said, a GalilDebug.txt file would be nice to verify this.
Steve