Not a networking guru, but sounds like routing? When a machine enters the cluster, is the routing affected? If I get a chance I'll take a look at how this behaves on our servers ...
We have a c7000 with two blades in a 2008 R2 HV cluster, iSCSI BE storage. We're using the Flex10 w/ 5 1Gbit SFPs for all network traffic (VM, blades themselves, and iSCSI traffic). They're in LACP ...