I ran into this last year and posted with not much response. The problem just seemed to stop.
We have run into this problem again, TODAY. All 450 + accounts have been getting locked out every 15 to 20 minutes after unlocking. As the day has drawn to a close, and workstations are being shut down for the evening - it's not happening anymore. We are starting to wonder if there is a workstation that is causing a problem/infected/hacked/something.
I have been scrounging for info/documentation or something to find out what is causing it - with no luck.
Have you had any success? Please let me know.
If we have this problem tomorrow, our plan is to do a little packet capturing. Followed by shutting down all workstations in the district. Then bring them back up, campus by campus until the problem reappears. Then we will have it narrowed to a campus - assuming it is a workstation that is causing the trouble.
'If at first you don't succeed, then skydiving isn't for you.'