Andem - 12:50 am on Apr 27, 2013 (gmt 0)
I think what you're experiencing is what many members here have been complaining about lately regarding loss of traffic. They have clearly stated that there are no manual actions taken against your site. It sounds like you're just not ranking well because their algorithm doesn't find your site useful for either visitors and/or Google themselves.
I'm not saying your site isn't useful, but they don't see the same value in it as you do.