Yes, Bob, that is possible, but unlikely given my latest discovery: I put in a wedge stem and started getting the exact same results! In frustration, I tore the stem out and unscrewed the headset top cap and yanked out the fork, and... the steerer tube has split like a tight pair of pants, about a 6-inch-long crack running down the slot in the threads along the spine of the tube. You can see daylight coming through. Mystery solved! And, if I can stop the next fork from splitting too, I can still use my stem. Glory be! Anyway, thanks for your help, everyone, that was the answer all along.

Of course, now where to find another fork that will fit the head tube on this 63cm bike...