If it does turn out to be the diodes, and for some reason you can't replace them then you can allways turn it into a pretty powerful and fast brushless motor.

Just remove the diodes, solder wires onto the armature and stator. Put a few volts on the armature and conect an ESC to the stator.By varying the current through the armature windings you can vary the torque - excellent for starting.

I did it a while ago and it seems to work well. I've not thought of anything useful to do with the motor yet.

Let me know if you/anyone is interested and I'll start a thread with more detail/photos.