Why has my page rank gone down?
 

Your back links would have gone down or removed perhaps. Check your Webmaster .

Check your backlinks status and again creates quality of backlinks.

Yap agree with above reply. Also may be your OBL has increased.

The key is to look at the quality of the site content is unique, whether other sites have done your mirror to your site link those stations, their PR value of the level of change will affect your PR value.

There was Google update that remove the Spam links , if you do not did this or your content of site is not unique due to this happens that your site PR is go down from 4 to 3

May be its due to your broken link check your site links.or your keyword competition is increased and your site is not getting sufficient traffic which is the major factor for the ranking.

To many backlinks all at once or from the same source will cause decreased rank of your website, and it's batter to have 50 quality PR4 backlinks then 500 PR0 and PR1 backlinks

no one can exactly know the reason, only our assumption are work like you said that lots of on page work are done in last 2 months i think it is big reason for over optimization or must your site must be heated by Penguine update..read about Penguine..

�The change will decrease rankings for sites that we believe are violating Google�s existing quality guidelines�this algorithm represents another improvement in our efforts to reduce webspam and promote high quality content" according to Google.

Re:
 

There are lot's of reasons behind decreasing page rank of any web page. You have to search reasons to drop page rank meanwhile you can start to increase back links with high page rank to increase page rank.