Rear index shifting not staying in middle gears I have replaced a chain (SRAM PC 830) and 7 speed freewheel (Shimano MF-HG37) on the back of my daily commuter bike about 300 miles/ 3 weeks ago with the same models.  The old parts lasted me 4000 miles, and until recently, the new ones have been fine.  Now I'm having issues of my chain not staying in "6" gear and jumping to 7 (the smallest).  I can shift to and from it ok, just that it will jump in a few revs.  All the other gears work fine.  If I move the shifter a little bit it will go back, but within a few revs jumps right back down.  

I have played with the high and low set screws since I'm also swapping wheels frequently too.  I had a axle break, switched to a backup wheel, went back to the repaired wheel, and then back to the backup now.  Through all that everything worked fine until the last swap.  The backup wheel is a identical replacement with the freewheel and tire transplanted.  

I tried to adjust the barrel nut along with the "b" screw.  Both had no effects, but I didn't move them very far.  I haven't found the time to try to clean the derailleur yet, but when I swapped the chain I gave the bike a good cleaning, wax, and lube bath.   I'm worried that I missed something in the derailleur or it just got dirty since.  Jockey wheels are clean and secure, but have 5k miles on them.  Front derailleur selections also had no effect.

The derailleur model is a Shimano Tourney found a 2008(ish) Schwinn High Timber with the gripshifters.  Laid down 3k miles ago where the derailleur was scratched and banged up the limit screws but no effect on performance.
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