intermittent trouble following VMs after vMotion

We're running Orion NPM v10.1.2, APM v4.0, IVIM v1.1 and our VMWare environment is ~90% ESX v4.1 with the remaining 10% awaiting upgrade from v4.0.

I've noticed that we periodically lose connectivity to virtual machines after they get moved to another ESX host.  The network adapters go into an "unknown" state and can't recover.  The original usually needs to be manually deleted and then we have to poll the VM resources again to get the "new", working vmxnet adapter.  Since our ESX environment can perform these moves just for load-balancing and without administrator intervention, it's a cause for some concern.  Has anyone else seen this?