This is strange. I put new wheels and tires on my 06 coupe and am now getting a vibration over 60mph. I can feel it in the steering wheel, gas pedal, shifter, and seat.

Here's what's really odd. It's not a constant vibration. It pulses. So, for about 1 second it will vibrate, then the shudder will fade until it pretty much stops vibrating; then back to vibration, then back to calm, and so on and so on.

Is this something that can be caused by improperly balanced wheels/tires? Is it a spacer issue?

The wheels are 18 in front and 19 in the back, like stock.

Any ideas?