Yep - Just did the repair and it was as stated above. Had exactly the same problem you have and changing this tiny drive belt was the fix. I add more details and another source to find this belt:
I had the same problem and a review of forums and my experience with my own fix indicated that this is almost certainly your problem:
The roughly 2.6 inch (circumerence) drive belt (that connects a tiny motor to the worm screw that moves the tone-arm laterally when it gets the signal) likely needs replacement. Mine was intact but old and rigid enough to keep slipping or hang up the tiny motor. Unplug the unit - take out the 8-10 screws to take off the bottom - remove this little belt - scrub the pulleys with tripled-up sewing thread, and replace with new belt. This is a good source:http://www.turntableneedles.com/Square- ... p_227.html
$6.50 (paypal available) for 2 belts to reach their minimum, free shipping, arrived in 2 days.
You might also want to clean and lightly grease the rail that the tone arm rides on, but this is unlikely to fix the problem without a replacement belt.