95 Explorer Sputtering at constant speed?

4.0L, and when I let the truck idle, sometimes it'll sputter once in a while. For instance it'll idle smooth for a bit once warmed up, then all of a sudden there's a miss, the rpm's drop, rev up a little, then smooths back out again. When I put it in gear and go to back out of the driveway, applying only slight throttle will cause it to sputter constantly unless I give it decent throttle. Then it runs smooth and pulls fine. While I accelerate, there's no sputter, but if I try to hold it at a constant 35-45mph, then it will start up again real bad. 55mph and up will run smooth also. What is my problem?