When a site gets put into the sandbox its traffic declines. WHen it is released from the sandbox the traffic may surge provided the site has the proper ranking factors. I have noticed that weeks or months following being released from the sandbox traffic and ranking will steadily decline. You get an initial ranking boost after being released from the sandbox and then a lot of your rankings fall soon after.
I have had that happening for over 5 years. The site's traffic and PR going up and down (bouncing between 4-6) and traffic very erratic. I haven't figured out exactly what it is.
After going out of the sandbox I may rank top 10 for a large set of keywords and after a few weeks those rankings slowly fall to page two or three.
Well that is common!! You need to have a little patience because it is like when you obtain high PR links which I know you do, it causes your site to go all over the place in the pages and it usually takes a few months to settle in.