Something else you can check is compare the volumes reported as having performance issues against the ones that are reported as fine and compare the performance statistics using the following command:
::> qos statistics volume latency show
Reference document: Display latency breakdown data per volume
Keep in mind the volumes you are comparing might have different clientside workloads, but in the above output mainly look for any major outliers. You can also perform a takeover of the node that owns the disks for the problematic volumes or an aggregate relocate and compare the statistics before takeover and during a takeover or aggregate relocate. This would essentially eliminate anything that is specific to the node hardware-wise.
Another check like the above would be to compare the statistics for any errors when the LIF lives on the home node and when you migrate the LIF to another node.
::> node run -node <NODENAME> -command "ifstat -a"
Reference document: Viewing or clearing network interface statistics
As @SpindleNinja you can open a NetApp Support case for a more thorough analysis of the Cluster.