Long story short, my BEST-performing sites with respect to Panda are those that I never touch. Old sites that are never tweaked/changed have fared well.
Funny that you mention this as the EXACT same thing has happened to me. I have a 6 year old site that always ranked well for its keywords for years and is one of my most profitable sites.
Since the Panda roll out in 2011 my site seemed totally unaffected. The site had remained pretty much untouched for about 2 years.
In February I decided to add some new content and touch up the header a little bit. On February 29th my conversion rates on my most visited pages literally dropped in half overnight and stayed that way. Then on March 21-23 my traffic dropped by about %30.
I also have another smaller site that did something similar. Had not been touch in a year and then I changed the template and reorganized some of the content and now I'm also down about 30% traffic on that site too.
I guess it could just be a coincidence, but it sure seems fishy.