There are lots of factors other than inbound links that affect page rank. For example, Google apparently likes to see the content on a page regularly updated. If it stays static, it can wither on the vine, so to speak.
1} Make your website w3c compatible.
2} Post your content in regular basis - so Google cached it fast it increase your regulations in Google eyes
3} Put less then 100 links in your 1 page
4} Put only the content what Google loves .
The decrease of page rank could be the result of losing quality backlinks from other high PR websites. Keep building quality backlinks is the only thing you can do to get your PR back.