Thanks for your responses.
Duncan, I've tried various amounts of pressure with no noticable change in results.
Mac, It's a 147 pound floor model, so I can't throw it very far.
About 3 1/2" actual throw, plenty for any pen blanks I've used to date.
The good news is the source of the problem has been identified, but the problem isn't solved, yet.
I called an emergency meeting of the NCWTG and one of my friends played with all the parts. As he was drilling his 4th blank with the same resuilts I was obtaining, I could see, from 6 feet away, the jaws rocking almost imperceptably back and forth along the axis of the clamp's drive screw. By holding the the screw with a couple fingers or by gripping the jaws with a free hand the wobble is reduced ( but not eliminated ).
This clamp is better than trying to freehand it, but it leaves a lot to be desired, especially at the price Rockler charges for it. It's a pretty tool, but it it's a long way from being an effective tool on its own.
Again, thanks for the help,