I agree with Steve. It's probably the kickdown cable. If you attempt adjustment, and the problem doesn't seem fixed, another possiblity is that the cable has aged and is binding, rather than springing back to its correct position, when you're off the gas. I performed a successful temporary fix by spraying some synthetic lubricant into the cable and moving it. Repeat this procedure a few times, and then try it. You'll have to take the cable loose from its bracket on the throttle so you can effectively get the lubricant down where it's needed.

The next step would be cable replacement.

G