rhythmic vibration

Well, we finally got around to looking at the harmonic balancer. A friend loaned me one from his spare 340. We took mine off, and there were no weights on the back. We put his on my car and ran the engine, and there was more vibration. The only difference between the balancers was where the "dimples" were located on the face. We put mine back on and ran the engine and the vibration was much less. We put everything back together and I drove home. At first, there was no vibration. I reappeared about 2 or 3 miles down the road. By the time I got home (11 miles), the vibration was back to it's normal irritating level.

I know for a fact the convertor is neutral balance. It's brand new, I put it on, and I still have the box.

We are having trouble understanding how the flex plate could cause the vibration. With so many bolts holding it on, how could it be "out of whack"?