bluntforce - 8:20 pm on Sep 21, 2012 (gmt 0)
There were Panda updates on April 19 and April 27, everything I've read about Panda indicates it affects perceived low quality sites, not one page. Penguin is about spam.
The question would be where was your traffic coming from prior to the drop vs. now?
Single keyphrase or many variants?
Do users stay on the page, or is it just a "landing" page?
Search engines are pretty good about understanding the concept of a page, they can then watch user behavior to determine user satisfaction with your page versus competing pages.
If you can't find any differences between then and now, and the user experience seems to be satisfactory, that would be the time to start making changes.