Din notice that any Google Penguin and Google Panda update recently . Can i know when your PR start drop ? Everyone just know that google penguin start killing over optimize website and duplicate content. Stay away from that in this time to stay safe.
check your backlinks and internal links. decrease in pr is like a domino effect one your baclink decreases your site will also loose some PR Juice.
This is probably because you may have created unwanted backlinks or may be you have sold out links to someone (outbound links). Check if you have done spamming in order to create link base. Remove the bad links in order to regain your PR.