weird motor problem on my cnc mill

I am experiencing a strange problem on my x axis motor on my cnc
mill. The motor is a Fanuc 0, 56V DC brush, 4 pole. One direction
the motor runs smooth, the other direction it growls and shudders.
I took out each of the four brushes and they look perfect. Peering
at the commutators, they look perfect too. The bearings *feel* ok
when you turn the shaft of the motor either direction by hand. I
ran the motor off of a 1 volt supply and it spins in both directions
without stalling.

On the mill, running it with a gecko g340 drive, the direction
changes from cw to ccw are harsh (but strangly smooth from ccw to
cw). At certain speeds in the ccw direction, there is a shuddering
of the motor, but not in the cw direction.

Any ideas what could be the culprit? If you can even find a used
Fanuc 0 motor on ebay, they run $200 to $500 in unknown condition...

Thanks for any help

Rick