'03 G35 Sedan
three months ago, I installed Z springs up front, left it stock in the rear, installed Tokico Coupe Struts all around and replaced the passenger side compression rod to get rid of a rattle.

After the work was completed I have a vibration over 60mph when cold and over 30mph when warm (after an hour of driving). The vibration shakes the rear view mirror, accelerator pedal and I can feel it through the driver seat.

rotors were replaced within the past year.

Since the vibration started I performed the following work to try to fix the problem.

Speed balanced the wheels
replaced the tires (needed it anyway)

Still get the vibration. Any thoughts on what it can be? I'm scratching my head.