Anyone encounter a problem in which the Guest OS in VMware Server randomly shuts down?

This has happened about once a month, where VMware Server just shuts down ALL guests OS. The host machine is fine, and you can log into the web management of VMware Server, it's just all guest OS get shutdown with no apparent reason. Even the logs say vmware guest os so and so was shut down, but no details why.

This is not just one one machine, we have VMware server installed on three different PowerEdge servers and they all have encounter the same problem. We've tripped checked all settings and nothing is out of the ordinary.