UMMM. Something either slippin or stickin.
Derek,
Have you checked the belt spindle? check to make sure its free of any grease or residue that may have gotton on it during polishibg etc.
Since the brush roll seems to be revolving with out a problem. I'd check the belt spindle shaft. Its unlikely, but possible the actual spinde shaft metal could be worn or even slightly warped after all these years, therefore causing the belt not to seat properly. The only way to confirm this is to remove the belt lifter plate, so you can actually see what the belt is doing when its turning on the spindle shaft.
You could try cleaning the spindel off with some windex and also try using a knarled brush roll belt. I'm really thinkin its more of a spindel problem, or the brushroll is just set to low for your carpet height?
Just my thoughts.