I just picked up a Durst M800. I'm having a problem (I think) with the head slipping on the track. It's strange, because it will lock if the raising/lowering knob is at anything but 12 o'clock & 6 o'clock. At 12 & 6 o'clock, it slides freely. I can't believe it was designed that way, so I assume it's a gear problem. Anyone have any experience, or a manual? Thanks.