Im not sure if this is client based but each client clings to the AP it first associates with, it will drop packets and never re-associate with the closest AP, the radio has to be turned off on the client, then back on before it will associate.
I am having the exact same issue but with ACS 3.3.3 on a windows 2003 server with service pack one. It works for a day or so.. then craps out. I even tryed new hardware... same issue. Just a generic base windows install