Hi,
Did this issue occur for all domain computer or specific one? Please check if the domain user profile is intact.
If you make sure there is no user removing the computer from the domain manually, I suspect the root is AD side.
Please check if you have set some schedule or anything else to clear some data from Active directory or DNS.
Use your anti-virus to scan the whole computer, then re-join the domain to keep observing.
Meanwhile, please refer to this thread:
http://social.technet.microsoft.com/Forums/en-US/aea7b73a-11e0-4021-8e8d-d08541fd2280/computers-falling-off-the-domain?forum=itproxpsp
Karen Hu
TechNet Community Support
