So... thanks
dsbrantjr for the reminder. I had a similar problem on a totally different set of shifters in the past which *was* related to a fraying cable. So I did replace the cable on this one, to no avail. In fact, the cable looked quite new. Seems like it could well be the issue that
AndrewRStewart mentioned. Is there a good guide available on servicing these, so I'd know how best to clean out any possible old gummy lube?