I'm just as stumped as everyone else, but have another data point to add: I have LP18's on my UO-8 and I've never seen a problem like the OP's.

In case anyone finds it useful, here are more details re: my rims & tires:
Panaracer Pasela tires, 27" x 1 1/8" (~28.5 mm width)
Pressures between 90-100 PSI
Velox rim tape