Hi,
Performance issues are often more complex than they seem at the outset. You haven't really said much about what the filer is doing: what sort of data, access patterns, volume/aggr setups, deduplication, reallocation, etc...
Without banging the "upgrade" drum too hard, I'd jump up to 7.3.5.1 . 7.3.2 had (as all ONTap software inevitably has) a few nasty bugs. 7.3.5.1 seems to running well for me on well-loaded machines with all protocols enabled, except NFSv4.
I'd also suggest you try running without NFSv4 as a bit of a bigger general test. NFSv4 is still not as mature and widely-used as it perhaps should be.
You might also want to enable logging of your domain controller connections (see 'options cifs' ) and some tracking of DNS response times, as well as performance on your kerberos server. Lags in completing authentication can cause these spikes.