Thanks for getting back to me.
I forgot to explain that this is a Windows Storage Server 2012 box, not an ESX host. But it's the exact same card with, what sounds like, the same issues. If it had the same problem with ESX then that indicates it's not an OS problem but either a setting required with the card, or an issue with our switch config, possibly cabling or transceivers that we haven't been able to find yet.
I can try going back to HP with your case ID and see if they can find something in there, but if you can recall how you fixed it, please let me know.
Cheers,
Liv