Don't the teeth on that wheel look pretty much worn out? I'm guessing that's the problem, but I'm as far from being a bike mechanic as I am from being a Ferrari mechanic, so feel free to enlighten me. The only other observation I can offer is that the pulley wheels don't turn as easily as I imagined they would. But then, the whole thing is pretty grungy so if I disassemble, clean, lube, and reassemble, that might go away. I'd rather put in a new pulley wheel while I'm at it. For what it's worth, the mechanic at the LBS seemed to think I'd need the same Suntour part for it to fit.
Thanks,
Dan