Do you have any switch logs that would show the switch detecting a loop and then placing a port in error/disable?
This sounds more like a config issue within the VM OS more than something on the virtual side.
Is it possible to assign multiple IP address to a single interface on your firewall VM instead of having two interfaces on the same subnet?
I assume you configured it this way to allow for different services to be hosted by the same VM?
I think the way it should be configured is two IP addresses from different ISPs instead of creating a spanning tree loop between your ISP and your Firewall VM.
Something to check:
If you do have a loop going on and spanning tree was saving you from it, you might want to check out your bandwidth usage of the external interface ports. You could be cooking your switch and NICs if they are maxed out at 100% dealing with loop traffic.
As for the KB I was looking for, here are the current bugs and fixes someone might see going into vsphere v6.0.
I think you said you are running 5.5 so this might not impact or fix anything for you.