I have seen an issue a couple of times now where shares on an Isilon intermittently appear offline, or if you have a share that is accessed constantly by an application, it can look as though they intermittently drop out. It can be tough to diagnose as browsing the share will show it is up and available, and when you call your network team to ask for help, you will be told there were no issues.
In each case I have seen the problem has been due to incorrectly configured Gateway Priority settings where the Isilon has multiple External Network Subnets.
As you can see below we have 2 subnets configure (I create these for the sake of this post so ignore IP settings etc)
When a subnet is created, by default the Gateway priority is configured to be 1.
What I usually do is set the subnet used for the management subnet to a lower priority
Hopefully this should resolve your intermittent drop out issues.