In my experience this has happened when there was an intermittent fault on a switch. I don't suggest this is your problem but is this happening in a particular area of a building or a specific sector of the wifi network? Is it happening to specific users only? Intermittent errors are always painfully slow to track down so one thing at a time, eliminated the easy stuff first. Sorry I can't be more specific, but this is where I would be starting.
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