Here's the prompt and courteous reply from Jason at Crank Bros.:

If the pedal threads are fine, I would check the cranks, they might be thin in the threads. I would also have a bike shop check it out to make sure everything is safe.
And, I think he's right. The threads on the pedal are fine, so somethin' funky must be going on inside the crank. These old eyes couldn't see inside there very well - it's off to the LBS, I guess.