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I would probably pull the tech data and open a support ticket. To do this: login to HiveManager, go the monitor tab, put a check next to the AP, click on utilities, and choose 'Get Tech Data'. It's best to do this as soon after you have noticed the AP has rebooted. When you open the ticket with support, there is a place to upload files. You'll want to upload the file you downloaded when pulling the tech data.
Are you seeing the lights reboot on the AP or is it just dropping off HiveManager? Any other changes made lately (Firmware upgrade, etc) that could help? Is it only 1 AP that is rebooting?