After monkeying with this all weekend, I have come to the following conclusion-

Aerating WHILE adding acid to lower PH is counter-productive.

I added more acid to get to 7 (actually just a hair below), waited until I got there then fired up the sump pump aerater. The next morning the PH was still 7, so I did a bunch of cannon balls and general splashing about, then left the aerater running while we were out of town. When we got back, PH was back up to 2, so I stopped all aeration and added acid to knock it back down. Started aeration and did a bunch more manual aeration, then checked PH again before bed, it was up to 7.3. Stopped aeration, added acid and went to bed.

Continued this process through Sunday. Got the alk down to 150, this morning PH was down to 7 again so I turned on the aerater again.

In retrospect, when I was adding acid with the air leak in my return flow, it apparantly was enough to prevent the PH from dropping low enough to get carbon dioxide production, so as the PH remained stable then rose, the alk went right along with it.

Now all I need is another gallon of acid and a few more days I have my fingers crossed I've finally figured this out.

Tredge, what did you come up with?