Whitey - 12:41 am on Oct 1, 2011 (gmt 0)

riding right along the edge of some sort of threshold

Sounds like it. In 2-3 months it took the first couple of updates, the time required to see a recovery, then in the following period it drops below the threshold.

Nothing was in done in between.

Sounds like more work is and was required. Add more content , keep it fresh, substantial and "useful", and work hard to get rid of duplicate pages and indexing paths, similar and/or thin content + usability.

Shatner - how many pages did you have, did you cut any out and if so what did you reduce it to? What type of content have you loaded on your site - is it aggregated ?