I don't think it is very likely that the capacitor is improving, BB - it is completely charged and discharged every time it throws (or tries to throw) a spark.
Did you carefully clean the surfaces of the points after you filed them? If you left any powder or oily stuff on the contact surfaces, you'd get pretty much what you are getting. I tend to pull a piece of thin, very fine abrasive paper back and forth between them to remove the burned metal, then I pull a piece of soft cotton cloth back and forth to remove the residue. The problem with filing is that usually the surfaces don't end up parallel to each other. I can remember when I was a teenager, filing a set of points and ending up with no spark at all. When I looked at them with a magnifying glass on the bench, I found that the two points were not parallel, and there was a thin grey line where their edges touched. Once I got them back to parallel to each other, they worked perfectly.
