All our servers (Windows 2008 R2) are configured to synchronize to the AD hierarchy. Sometimes the time source of the Windows time service is reset to Free-running System Clock. This happens as far as we could track it, after a reboot of the server.

After it was reset, you can synchronize with the AD again:

w32tm /config /syncfromflags:domhier /update

But some days / weeks / months (we couldnt really find out yet, when this happens excactly), the time source is reset again.

I already tried to /unregister and /register the time service, but this changes nothing in behavior.

All servers are virutalized. I already checked, that time synchronization with host is turned off.

Also there are no group policies, which set the time service configuration.

What else can I do to find out, what causes this configuration reset?