Hi, I too am having problems connecting netbooks to the DWL-3500ap's. There are upto 30 netbooks in a class. Some classes have 3 WAPs providing plenty of coverage but they still get limited connectivity. I recently updated the firmware on all of the WAPs to the latest version 3, as recommended by my supplier and D-Link, but the problem persists and seems to have gotten worse! I haven't updated the firmware on the managed switches though. Did as you say disabling the Spanning Tree Protocol fix the issue permanently or do you still have trouble? Did you disable it on all managed wireless switches?
Thanks,
Nick