NFS is notorious for these type of issues.We had similar problem where it did not PSOD but the VM were powered of as ESX server was too busy trying to restore NFS
Did you mange to find the issue as im having the same problem with my servers?
Well we've resolved our issue, hope the same has happend with you.
This kB has helped. http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2239