There is a bug in some place of Ubuntu 10.04 that does crazy things with virtual machines using KVM. Symptoms are a inconsistent cron process and time moving backwards. I didn’t know the root of the problem but I suspected that it was related to power saving. Why? Because the problems appeared at night, when the systems are usually idle. Solution: processor.max_cstate=0. That disables CPU power saving. I don’t have an entire explanation, but that has been working for me.