Has Your PageRank Dropped Unexpectedly?
There are plenty of reasons why PageRank might drop. But one of the chief reasons you could see a drop in PageRank is if you write to your blog less often. I have a blog that I wrote to daily for a year and within 6 months I had achieve a PageRank 5. After one year of daily blogging I scaled back to writing just when I had the time. It averaged out to a couple of times a week. Within 6 months my PageRank had dropped back down to a PageRank of 4.
This is an object lesson. You should write to your blog daily. It will increase your credibility and authority. Anything less than daily blogging will provide fewer benefits.
Notice that the quality of my blog posts did not decline when I wrote less often. The quality has always been high, but now I write less often. That’s a key indicator for Google that your blog is less authoritative than your competition’s is. Don’t let them think that about you. When Google decides that your blog is less authoritative, so will your readers. That’s why you need to write every day.
Professional Management for Business Blogs
About the Author: 
I am not sure I agree to that observation. I have two blogs that updated regularly and haven’t updated them in a year but their PR (3) is still intact.
I can’t think of any other explanation. I’ve made no other changes to my blog. It could be that Google’s PageRank algorithm only affects blogs with a higher PR. I have noticed that some blogs I manage that have only attained a PR3 have not gone above that level and they aren’t posted to every day either. Maybe Google has a PR cap for websites or blogs that aren’t updated as often.