I have a similar strange vibration problem too. Mine occurs around 70-80 mph - give or take 5. The strange thing is that it isn't a constant vibration. It happens every 3 or 4 seconds. It seems to start at the front of the car, and kinda like a shudder - passes back through the car, almost like it is going out the back. A few seconds later it happens again. Over and over.

I figured it was my tires. My rears are Sumitomo's and they're shot (the center 3-4 inches of tread is bald). And my fronts are Advans, and the inside edges are bald.

What would cause just the inside edges of my front tires to be bald? Does it have to do with the vibration? I dunno. I'm getting new tires in a couple weeks anyway.

What is road force balancing that you guys mentioned?