From a previous post by Euroman:
"Try to isolate whether it is an ignition problem or fuel problem. Are the spark wires relatively new. If they are the originals, the metal connectors can go bad and give a rough idle but the ignition will provide enough spark to go through them at higher rpm. You could pull a plug and see if you can tell which one is discolored. Sometimes you get a bad one or it gets a cracked insulator on installation. If one is darker than the others replace it and see if that corrects the problem. If not check the spark wires with an ohm meter. You should have continuity with resistance of about 2K ohms or less."
The full thread is at: http://www.peachparts.com/ubb/Forum1/HTML/005266.html
I'm not a tech, but I did have a similar problem with a rough idle that smoothed out on acceleration, and a new plug wire fixed the problem, so take it for what it's worth I guess.