Time for a new cassette?

So, I recently changed my chain for the first time ever - big win

I re-indexed the gears and on the work-stand they are perfect - big win

When riding under power however I get an occasional skip on the 11 tooth cog on the cassette, that gives me limited confidence in pedalling in this gear. Happens with both chain rings so is not chainring related... all the other cogs (from 32 teeth to 12 teeth) are fine.

Is this a sign my cassette is on the way out and needs changing? (still haven't found a good LBS yet as you may tell) or do I need to check my indexing again (have done so 3 or 4 times now and every time on the stand its perfect, its only under the strain of actually pedalling it has the odd skip).

(Sorry for turning this into the Drew seeks advice about his bike issues page :D)