this days Google updated PR of all sites. 70% of the sites have changed. and again Google prove that we are wrong about PR and the way we understand this parameter. some of you will notice a drop down in PR even when they have build a large quantity and good quality of backlinks. others will notice a major increase even if they do not build a lot of links. My sites for example changed according to the age of the sites, the old ones have lost PR value and the new ones have increased the PR value. My main site, and this one I promote hardly building hundreds of good quality High PR backlinks to it have lost PR but at the same time get better ranking on the SERPs position. One of the newest blogs I have, It's a few months old has jumped from PR0 to PR2 with only 300 to 700 backlinks So again I think that we do not need to pay any attention to the PR value but to the rankings.
Maybe the problem is that you are using an auto tool for seo (second link in sig), we have found our sites reasonably stable for the past few months rising constantly but all on page 1 of google, but we do not use any automated tools.
Yesterday, my site's pagerank reached from 0 to 4 suddenly. Dont know how. I have not followed any link building services yet.
Having an unique content? Well the latest PR mostly ranked up those sites whose having a strong content rather than backlinks. That's why Google installed panda and observe the sites taking a long time to update the PR.
my article directory which i think has no original content gained pr2, So exchanging links with related site still works.
same as you my old site pr have come down why it has come down i dont know... i have doing link building regularly.
I think they have changed some algorithm again. As you said my old site also lost PR and new site gain PR.
All of my sites stayed the same or went up. The weird thing is that a stupid site I've had for about 2 years with no real content and all images went from PR0 to PR3. I've never done any link building for this site. It's just a Hot or Not clone site that I put up for fun one day. PR3??? Okay.....