Rough geartrain

This is the motor out of a class 141 locomotive. I have found that the gear train gets very rough in certain conditions and run as sooth as silk at other times.
If I hold the motor with the armature cover side down, the mechanism turns perfectly.

If I turn the motor over like this:

then the gears jam and are very rough to turn.

It seems to be related to the lateral play in the two axles. If I place the motor (as it is above - i.e. without armature/loco housing etc) on some track and push it along it runs nicely but as it gets into a corner (thus pushing the axles to one side) it jerks and stops.

The gears do not look worn at all. No amount of cleaning and lubrication seems to help.

What is the problem and what can I do ?