I've also run into performance problems on 8.1.2 (no patch) in 7-mode, but when running a FPolicy connected to a CommVault Archiver modules, so that stub files get re-hydrated properly. If we hit it too hard with too many requests, the NFS performance goes into the crapper. When you don't remember you have FPolicy enabled (because it just works usually) and there's nothing in the logs or in sysstat showing why NFS is pausing or just stopping, a reboot has worked.
But when it happens a second time in a weekend, you start thinking and remembering more and turning off FPolicy brings performance back in an instant. I suspect we're both running into the same type of issue, where NFS gets pushed down the priority stack, or it just has too small a buffer to handle outstanding NFSv3 requests or something.
John