Dave_Hybrid - 5:16 pm on Nov 5, 2013 (gmt 0)
Interesting... I've heard a few reports like this now.
I redesigned a site 2 years ago and it tanked, never recovered. It coincided with a panda date but I can't be sure it was that.
Anyway it has 50.000 404 errors due to an old plugin. I recently 410'd them and have also started marking them as fixed in webmaster tools, 1,000 per day. It's slow but they are going down.
Did you do anything about the 404's other than the above?
Congrats on the recovery.