So, I've done a complete restoration/overhaul on a mid 90's Colnago Master with 8-speed Record. Everything works great except for some minor issues with the rear derailleur that bugs me. Something like a third of the time, when shifting from the smallest cog to the next, and in particular, from the second smallest to the third cog, the chain won't shift, it just stays in a noisy middle position. It only happens when making single shifts, two or three gears at a time works perfectly. I've obviously tried putting a tad more tension on the cable but this only results in noise and overshifts, so it doesn't seem like cable tension is the issue?

The cassette, cabling, chain and pulleys are all brand new so shouldn't be a wear issue either.

Any ideas?