IOS has got nothing to do with the issue.
I've noticed that the WAPs are 1141, which means that the clients are using 802.11b radio. Have you checked for channel interferrence?
What are the signal strengths of the clients as seen by the WAP (Command: sh dot11 associate )?
What is the signal strength of the WAP as seen by the client?
When you say disconnect, can you be more specific? When the client "disconnects" does ping drop? Or is the client claims of "disconnect" on the application level (cannot receive email BUT can still browse the web)?