What adjustments were you making to the mini playfield?  I have two Dr Who at the moment and both aren't as smooth as I would like.

First thing I did was to dismantle the mini playfield completely. I removed the old grease and replaced the shims - all four of them. Check the horizontal plate with the four screws. Two holes are elongated on the guide, and this step is crucial. This governs the up and down movement by bringing the two arms closer or further away from the moving carriage. I adjust this while the motor / arbor are removed as you can feel the physical resistance by moving the mini playfield up and down manually. Once you find the least amount of resistance, tighten the four screws and then check again. Then you can mount the motor - The motor / arbor must be parallel with the moving carriage and this is the key to a smooth moving carriage. I attach a 12 volt power supply - observe the movement and make the adjustment.

Once this is done, I place the entire assembly in my vice and then connect 12V to the motor. The Mini playfield should move easily. Then place it in the game and be sure all the wiring is free from obstructing the movement of the mini playfield up and down. By this stage, you should have a perfectly working mini playfield.