2_i 

The "skating" when riding with the largest ring turned out to be the residual problem with poor shifting to the middle cog from the smallest cog that I hoped to have solved with cleaning of the pulleys. (The latter helped but evidently not fully.) The problem that developed could be best illustrated with the diagram describing shifts: 12t -(pathologically hard)-> 14t -(pathologically easy)->16t and 12t <-(normal)- 14t <-(normal)- 16t. If tension in the shift cable was too low, the derailleur was failing to move the chain from 12t to 14t. This seemed tied to an insufficient stiffness in the plastic derailleur yielding hiccups in the derailleur operation. If I increased tension in the cable, the derailleur immediately started to overshift, jumping from 12t to 16t for the middle shifter position and then falling back to 14t, and so on, yielding the "skating".

I could not find a middle-ground cable tension that would solve the problem and I went with a nuclear option. Given that I had cassette pieces for all kinds of speeds, I reduced the gap between 12t and 14t, by using 10-sp 12t cog with built in spacer, and I increased the gap between 14t and 16t, by using a 9sp spacer there w/o filing. In my estimate one gap was reduced by just 0.15mm and the other increased by that amount. This nuclear option took care of the pathologies (or basically went along with them) and all shifts fine now for all gears in all combinations. In the back of my mind I worry though that, if 0.15mm matters, then with temperature changes or something, and the derailleur arm softening or stiffening, the problem might come back. If you do not try, though, you will never know .

The bike is now all ready for a trip in 2 weeks from now, where my survival crucially depends on this bike - my lodging location will be inaccessible by public transport for much of the week. The bike better perform there.