Rear derailleur shifting poorly, skipping gears I've got a Shimano Sora on a 8-gear cassette, and have odd behavior and skipping, shifting under stress. 

This started with a trip to the mechanic for a tune up which replaced the chain. I had some skips so I replaced the cassette. Still more skipping, a second trip to the mechanic didn't do much good.

I tried adjusting it myself and found some odd behavior:


Had to adjust the H screw way out, and it doesn't seem to reliably return to the same spot while pedaling (while working on it).
When I start at the smallest gear and shift to the largest, everything looks perfectly aligned.
When I start at the largest gear and shift to the smallest, the first click does almost nothing. The second goes to the 7th gear, third click to the 6th... and the seventh click skips from the 3rd gear to the first.
After all this, it doesn't seem to move back into the original position without a little push.


Any ideas on what's going wrong here? It seems like tension pulling the chain out is not sufficient, but I don't see any adjustment for that.

Am I barking up the wrong tree?
A tune-up fundamental SHOULD be the proper alignment of the rear derailleur hanger. If the shop doesn't use a Park Tool DAG-1 [derailleur hanger alignment gauge], or some other manufacturer's, to inspect, verify, and correct the relation of the hanger to 3 different points on rim of rear wheel at 180 and 45 degrees apart it is throwing parts at a bike. As the hanger should hold the rear derailleur in direct line with each cassette chain ring, any deviation -- inward or away from the cassette -- induces erratic shifting and skipping that mimics cassette and chain wear.  (Misalignment is difficult to "see" without this tool but may be suspected if the bike has been bumped in a rack, dropped, or fallen, on the drive side.)  The tool at $65 to $75 will be used for diagnosis by a careful shop but is unlikely to be worthwhile for an occasional home wrench.