In my honest opinion you seem to worried about nothing and asking the wrong question
Don't worry about PR - review your traffic stats and conversions, that's what MATTERS and PR is not all THAT important for you to worry about.
PR fluctuates and a decrease IN PR is not by definition a decrease in traffic.
And forget about Alexa, just remove it from your list of possible concerns to your business/online endeavors.
Google is known to show irregular results for your backlink profile.
You're better off using
Yahoo site explorer, this gives you much more accurate results.