2001 Lincoln Town Car VIBRATIONS

WHEN DRIVING AT ANY SPEED I GET A INTERMITTENT VIBRATION THAT DOESN'T HAVE A PATTERN
TO IT AND FEELS LIKE GOING OVER THE BUMPS ON THE SIDE OF THE ROAD, IF YOU KNOW WHAT I MEAN. I THOUGHT IT WAS THE AIR BAGS SUSPENSION, SO I PUT ON NEW AIR BAGS, STILL HAVE THE SAME VIBRATIONS AND IT SEAMS LIKE THERE ARE CONING FROM THE REAR OF THE CAR. THIS IS NOT A STEADY BUT INTERMEDIATE VIBRATION. ONLY LAST MAYBE 3 TO 4 SECONDS AT A TIME. AND SOMETIMES 10 TO 15 MINUTES BETWEEN VIBRATIONS. WHAT ON EARTH COULD THIS BE.
THANKS FOR YOUR RESPONSE. ROB