Good morning!
So I just finished at a client site where they did a fresh install of vSphere 5.5 direct-attached via FC on UCS to a NetApp 2240 running 8.1.2 7-Mode. Their environment was performing terribly and was unreliable. VMs would seem to function, then become extremely slow and seemingly unresponsive. No errors were found in the logs of the NetApp or on the UCS. I saw log entries similar to yours on the hosts, though. What versions of vSphere and storage array are you running? What events are you seeing on the Tasks and Events tab in the vSphere Client?
To my surprise, it turns out ESXi 5.5 is not compatible with any version of NetApp Data ONTAP below 8.2P3. Even then, I was even more surprised to see that at that code version, NetApp doesn't support vMotion or HA with ESXi 5.5.
So I left the NetApp code at its current version and downgraded the hosts to 5.1 to get to a supported state. The errors went away and performance is great - everything works as expected.
In short: check your code interoperability. You might be surprised!
Cheers,
Mike