For what its worth, I have been following this and one of your other posts, as I was having very similar issues. Nantoons was quite help ful there as well, for me my issue, as best as I can tell is that the pots I had were garbage, thy were new, or at least as far as I know, bought them Very cheap of ebay. so i need a different pot for another project, so I had gotten some more, and it seems like that was my issue! and the strange thing is that i got the same stable results from a 1k and a 20k pot so that answers my question if it maters on the pots resistance, as I was using 5k pots, and though maybe I needed 10k only because it seems like that's what is in many tutorials, I do think it would be a good size to use though.
In one of your questions you asked if you should solder your midi port, in my experience, any wiggle room will send all sorts of garbage thru so you may want to do that or if you are still needing it to be in the bread board, find some nice tight holes if you can and if it seems suspect you can always wiggle it around a little. and as a side note, I am using slightly modified version of your other code, combined with a couple lines of nantoons, because, I picked up a midi shield to eliminate wiring problems, and then realized that i still had the same issues, but, that it was the same shield that nantoons was using.
Good luck, and post your results
HANKENSTIEN