There was another thread on this recently, and the bottom line is Alexa is getting less and less accurate over time because your rank depends on people visiting your site with the Alexa toolbar installed. As fewer people use the toolbar it's going to adversely affect the traffic for practically all sites in some way. I wouldn't worry about it.
My site's traffic has increased by about 1,000 visitors per day in the last 3 months yet my Alexa rank has gone from 35,000 to 50,000.
I use PageStrength
http://www.seomoz.org/page-strength
Alexa sucks!
Linear Mode
