Everything went back together great with new chain, shifters, cables and 25 years of dirt and grime removed. The ride is night and day compared to before the service which I knew was long overdue. I got the shifters adjusted nicely and everything is good there except for one issue that was present before and I was hoping would go away after all this attention. With the chain on the middle sprocket in front, all gears at the rear perform fine except for #6. It is finicky to shift into either up or downshifting and if I do get it engaged, it is still not solid and any hard pedaling will cause it to become unstable and basically be in loose engagement.
Every other gear is rock solid and I can stand on the pedals in any of them. They all shift smoothly too. I also shifted the chain on to the small sprocket in front and got the same behavior. I did not try it with the chain on the big sprocket in the front.
I thought with all of the service it got, this issue would go away but if anything, it got worse. I looked closely at the cassette and can see no irregularities. Please pardon me if some of my terminology is a little off.
.
Again, any input is greatly appreciated