X axis vibration

I have a Bridgeport sers. 1 with a Crusader 2 control. It uses SEM brush type servos with Glentek servo drivers and glass scales for position. The problem is that the X axis vibrates and gets VERY HOT when stopped. I have set the signal and balance as per Anilam instructions and it is a bit better but if you place a load on the hand wheel it will start vibrating and then wont stop. Swapping the drive cards or servo motor the problem stays with the X axis. Removing the feed back conections no difference. The Y and Z axis are fine they make the normal "singing" noise when stopped. Anyone have any ideas were to look next?
thanks Bill

Usually the gain setting too high.
Al.