If you hold the rear off the ground (by putting the bike on a stand, for example) and pedal fast, what happens when you stop pedaling? Does the wheel keep on spinning fast for a good amount of time, or is there immediate slowing? Any grinding or rubbing sounds? It could be the freehub is on its way out if not a more obvious problem. You may not notice this much on the road except the wheels won't spin up quite as well as they used to.