You may be facing some local disk problems. Add a vMA to your environment and use it to collect the logs from the ESX servers: http://kb.vmware.com/kb/1024122

Also I am quite sure there is a syslog server included on vCenter installation. Use it if vMA does not work.

I had a very simmilar issue where the KB http://kb.vmware.com/kb/1030265 resolved the issue. You can try on a test ESX to check if this makes the problem stop.