What type of storage are you running? If it is by chance NetApp, be sure to check out NetApp Knowledgebase - How to troubleshoot NFS APD (All-Paths-Down) issues on VMware ESXi - I was hit with that when I ran NetApp.
Do you have multiple hosts, and if yes, are they all affected?
Do you have multiple datastores on the storage, and if yes, are the all affected?
How frequently are you seeing these (hourly, daily, weekly, etc.)
Do you have LAGs setup to the storage?
Hopefully we'll be able to help you out on this.