Rear index shifting not staying in middle gears
Something to keep in mind, it's not unusual for a SRAM chain to not play well with a Shimano cassette. A lot of times you'll get lucky and the two will work well together, I used to run SRAM chains with my Shimano cassettes all the time without much trouble. But there are just some combinations that just cause problems, even intermittently from run to run.

Or, you said you gave it a lube bath, I wonder if you managed to displace the grease inside of the derailleur.

When my rear derailleur just won't cooperate, I use my two favorite articles to troubleshoot and properly lubricate and adjust it. Study these thoroughly and meticulously and you'll likely happen upon your solution: 

http://sheldonbrown.com/derailer-adjustment.html

http://www.parktool.com/blog/repair-help/rear-derailleur-overhaul