I have on my commuter bike a set of 8-speed Shimano 600 (circa 1992) STI shifters and deraillers. Lately, for the last year or so but getting worse, I can't get the indexing adjusted so the shifting is smooth in both directions. What happens is I can get it to shift smoothly going down (towards the big cogs), but then towards the middle of the cluster going up it will get stuck and I have to shift up one more cog and back-track to get the derailler synchronized. If I turn the adjuster barrel, I can get the upshifting to be smooth, but then again in the middle it will get stuck and won't downshift unless I downshift one more gear and backtrack up one to synch the derailler. I can't seem to find a sweet-spot that works both ways.

I've changed the internal cable, lubed the guides on the BB, adjusted the B-tension, all to no avail. Could the shifter just be worn? I have done the de-gunking of the internals which these shifters were known for. My next step is to change the cable housing. If this doesn't work, maybe it is time to get some new shifters.

If the shifter is still consistently clicking through the same number of stops, it is unlikely to be the shifter's fault. The cable position is decided by a discrete number of teeth or détentes, and there is very little wear possible that could effectively move the position of the détentes.

From your description, I think the two most likely culprits are cable friction (it doesn't make much sense to change just the inner wire as the housing is usually where excess friction is caused), and derailleur hanger alignment. Other possible causes are a very worn chain, or a derailleur that is worn or damaged to the point of being sloppy.

Smooth shifting one way but not the other, and the ability to change between them with the barrel adjuster, is a classic indication of cable (and less often derailleur) friction. New inner wires often do not play well with a used housing. The wires tend to wear a groove in the housing (a major component of cable "stretch") and a new inner wire may not follow the groove in the same way. If you change the housing do not neglect to change the last loop going into the derailleur; water, rust and road gunk tends to accumulate in that run since it is at a low spot. After replacing the cable, and always, make your adjustments gradually so that you do not adjust past the ideal spot.

Checking the rear derailleur hanger alignment is a first step in diagnosing any rear shifting problems.

I replaced the outer housing, and now the bike shifts perfectly! Thanks for the help.