Planet13 - 3:26 pm on Jun 5, 2012 (gmt 0)
penalties can be keyword specific (algo changes can be keyword specific, too)
example from my site:
blue widget (singular) ranks well
blue widgets (plural) doesn't rank well
Could be a manual penalty or could be Penguin.
Only way to find out if it is a manual penalty is to file a reconsideration request.
Hope this helps.