odd engine miss (i think)

[skilletky]

truck is a 98.5 24 valve. when the truck idles, it sounds like it has a weak cyl and runs kinda rough, but under load, it runs ok. hooked up my scanner to do a cyl cutout test. every time i would shut off a cyl, didnt matter which one, and then turned said cyl back on, then engine ran smooth as can be and sounded good for about 10 to 20 seconds. then it was back to idling rough. truck has 320K miles on it. VP looks to be newer, but have no other info on the truck. when checking the sensor data with the scanner, it shows 11.6 volts for the injection pump voltage. i checked battery wires, and the wire harness, but found not issues. unhooked connector at ECM and the VP, no corrosion or bent pins. no codes active or inactive. is the VP computer taking a chit?