Each and every time I've seen this behavior in a dog, it has ended up being an anal gland issue coupled with a foreign object passing through the system. Usually the dog will start chasing their tail, biting at the tail or rear though usually since their is pain in the anal gland area, they don't actually bite there but around the area and on the tail. They do a lot of digging and vocalization as well. I know you have stated that you don't think this is the cause. It is worth at least checking to see if this is an issue.
I would go ahead and add a little mineral oil to his food (a spoon full). This will help if there is a foreign object or an overly dry piece of fecal matter moving through. You can go ahead and check the anal glands and see if you can express any fluid from the glands. To empty them or express them, you will want to cover the area with a tissue and press your finger on the dog's anal glands with an upward motion. A foul smelling liquid should come out. If nothing comes out and your dog appears to be in pain, you should take him to the Vet. Your vet can do this easily, as he is experienced at it.
Excellent site on anal glands
You can also try applying a warm compress to the anal gland area to help ease any discomfort there might be.
This behavior may also be related to seizure activity. Seizures can manifest themselves in many different way, so this does have to be taken into consideration as a possible cause. You can read about seizures here playing attention to focal seizures.
If the behavior continues, and an anal gland issue is ruled out, go ahead and record the behavior with a digital camera, video phone or camcorder so you can show your vet.
One other thing you might want to look for is ant bites or stings. The fire ants are an issue this year and they can definitely cause bizarre behavior when they have bitten or stung an animal. Additionally, the bites continue to cause discomfort and itching long after the insect is dead and gone. The bite is usually red with a little circular ring, sometimes raised, but not always. Benadryl will help with the itching, but is only temporary.
I hope this information is helpful to you.