I've had 3 tubes go flat on me in the last 7 days. All on the rear wheel of my Super Course.

When I inspect the tubes afterwards I see a tiny hole about 6 inches away from the valve along the part of the tube that sits on the rim strip. I've checked and re-checked my rim and rim strip and the inside of my tire (Hard Case Bontrager's) and I can't find anything even remotely spikey in there. No spokes sticking up through the nipple, no burrs along the spoke bed or on the sidewalls...

The only thing i've noticed is that if I park the bike with the rear tire valve at the 6 o'clock position, the tube goes flat. If I'm careful to park it with the valve at the 12 o'clock position it won't go flat.

any ideas?

I'm about to just bring the rear wheel to my LBS and say "fix it", but my ego is not yet ready to concede defeat.