Problems indexing gears
B-tension issues rarely pop up spontaneously. Once it's set, you usually don't have to ever touch it unless you change to a much larger or smaller cassette. Unless you messed with it already...

I see the scrapes on the derailler and think your DR hanger is probably bent. This can cause irregular shifting issues like you mentioned-- good in the lows but bad in the highs, or vice versa. 

Or, your cassette/ chain might be worn out. Do you usually ride in the big cogs? If so, those will be the first to wear out and they might not shift right, while the unworn smaller cogs still have the sharp ramps to shift well.