I 100% agree with EchoMac. Please have your vet refer you to a specialist (veterinary internist).
I am an emergency veterinary nurse and assistant to our internist, neurologist and surgeon for 8 years now.
I think it is your next step if your vet(s) can not find an answer. And this doesn't mean your vet is stupid or anything.. sometimes they just do not have the right equipment or experience (meaning specializing) at their office. If simple metronidazole isn't helping, I would seek aggressive diagnostics.
If your pet is doing better (I hope) since this post, then that is great news!