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I have a Windows 2000 Server SP4 guest running on a Windows Vista Ultimate SP1 host.

Most of the time it works beautifully.

However it goes through periods when it will suddenly move the guest clock forwards or backwards by 15 or 20 minutes, and then correct it again a short time later (usually practically instantaneously - I've only actually managed to observe it directly once). This might be repeated several times over a period of a couple of hours, then be fine for a few hours, then start again.

The host clock is synchronised hourly to an internet time server. I have no evidence to suggest that the host clock is changing.

This is playing havoc with a program that runs on the guest, collecting continuous realtime data (anything up to about 200 data points per second). The timestamp recorded with each data point is critical, but these clock flips render them useless.

Has anyone experienced anything similar, or have any idea as to what might be the cause?

Do you have the Guest Additions installed? That should keep the Guest time in sync with the Host.

Yes, the Guest Additions are installed, though the service fails to start properly, logging the error 'The service did not respond to the start or control request in a timely fashion.' This happens even if I try to start it manually once the VM is up and running.

However apart from these occasional transients, the clock synchronisation is perfect. Correct me if I'm wrong, but I would have thought that if it were simply that the Guest Additions aren't running, there would be a gradual drift in the clock rather than sudden shifts that are almost immediately corrected.

I should point out that I haven't entirely ruled out the possibility that there is a fault in the host, but I'll run the same data collection program on the host tomorrow so it will soon be clear if there is.

It's also worth observing that I would never have noticed this problem if it weren't for my data collection program: there are no other detectable consequences. So it could potentially be a more common problem that just hasn't been noticed.

It's possible that the GA are responsible for this. Perhaps along with the NTP updates of your Host.

Sasquatch, please could you clarify.

Are you saying that the GA could be causing this because the GA service is not running properly? - in which case I need to find out why.

Or are you saying that the GA may be responsible regardless of whether the GA service is running properly? In which case, I guess I'll just have to give up on VB for now.

Though I could turn off the NTP syncing for a while to see if that makes a difference.

I'm saying that even though you may think that the service isn't running, it might actually be running without any problems. Do you have seamless mouse integration and custom resolutions? The service is broken since a long time, it never started or ran.

And that due to the timesync of your Host, the Guest Additions' TimeSync might cause this strange behaviour.

Oh right, that's helpful. The mouse integration, custom resolution etc is all flawless so I guess the GA must be running ok.

So tomorrow I'll check it out without the NTP syncing.