mysterious persistant scratching - need HELP!

[dutchilicious]

I *really* need help trying to find a solution to this. I'll try to be brief...

My 5 yr old dog began scratching at his left shoulder about a year ago. The "scratch" is pretty aggressive, ONLY happens during activity (about 15 minutes into it), and is ALWAYS the same spot on the left shoulder. Once the scratch starts, it persists until he stops moving. Think: 20-30 scratch-fests in a 45 minute walk. He is not otherwise painful or lame. There is no or visible irritation.

We have seen 8 vets (and counting), including a host of specialists (derm, ortho surgeon x3, internal med). We've been to a veterinary school/teaching hospital. All exams (derm, ortho, neurological) have shown nothing. We even had his microchip removed, as it was located *exactly* at the scratch spot (there is still a 2nd chip in there, not very far away from the spot, that hasn't been proven 100% innocent).

Still -- scratching!

For lack of any other answers, behavioral issue is the #1 suspect (Obsessive Compulsive) but that diagnosis just doesn't fit. My dog will stop to scratch while running around the beach with his dog friends... will stop mid squirrel chase to scratch... will sit down in the *middle* of crossing a busy city street to scratch. But when sitting around at home, inactive, never scratch. Just doesn't seem very OCD to me.

(sigh)

We're waiting on further recommendations from vet school. In the meantime, I'm sending out as many SOS's as possible. Just in case someone stumbles on this who has experience with this sort of oddity.

THANKS!!!