Coolfrontier

Bought a new Frontier in Sept 02; have had a "sprung spring noise" and vibration that occurs frequently but not predictably. When I say frequently, I mean sometimes its once or twice a trip. The vibration can be felt in the pedals, shifter, steering wheel and seats. It happens when braking or accelerating, once even when completely stopped. ABS light usually comes on at about the same time; but not always. This problem has been intermittent since day two. Its been in to the dealer twice with no luck. Now has 8k miles and still "springs". Can't tell if its coming from the front or rear, doesn't pull to either side. I'm leaning towards a possible transmission problem, but am very confused and looking for possible sources. Any ideas welcome.