Jamming Axis in rapid

Hi
I am having problems with our old Bridgeport Series 2 CNC milling machine. (pre Interact!)
The transistors blew on both X and Y.
I have managed to replace X with no problems and can get Y to move n feed rates but as soon as I try to move it in rapid the transistr blows again!
I did notice a sudden seize up when moving in rapid in a diagonal movement.
Could this be simply a lubrication problem.
After the slight seizure the Y axis moved again in feed but once again blew in rapid.
Any input appreciated