vibration...but not always

vibrations.....gotta hate 'em.

OK...i have a strange vibration thing going on that i have NEVER seen before....only at 55+ i get a vibration sometimes. i notice it most on the interstate, about 65-75 mph. the car will be fine, and i can hit a rough spot in the road, and the car will get this terrible vibration.....a fast vibration, not a rumble. then it will go away after a couple of minutes....or it will go away if i goose the throttle and let off......until i hit a rough spot again.
i thought maybe tranny mount, so i made sure it was nice and tight.....all's good there.
It seems to have started when i put on the S/S springs, because i never noticed it before.

oh, and i can feel it MOST in the seat. and sometimes it starts even without hitting a bump or rough spot, but mostly when i do.