tedster - 12:20 pm on Jun 6, 2013 (gmt 0)
First, understand that it's impossible for anyone outside of Google to give you the "exact cause" for a ranking drop. Long before Penguin and Panda webmasters were already seeing mysterious drops for one keyword.
1. I would double check ANY actions you've ever taken (on-site or off-site) that are specific to keywords - really look hard to see if those words are used naturally.
2. PR0, if you've never been higher, means you have very low backlinks, either low in number or low in quality or both.
#2 means to me that you need to make a long term project out of brand building. It's really tough to improve rankings when your brand is weak.