Truck stumbles/hesitates under power.

This manual transmission has probs when accelerating. Idle is ok. When stumbling, shifting into neutral stops the problem. On level ground, at constant speed, say 55 mph, car runs smooth, but trying to accelerate will start the stumbling. Foot off the gas ends the stumbling. Same exact symptoms when using Cruise Control. Using Cruise Control (CC) on level road at constant speed works fine. Approaching hill, where CC attempts to accelerate to maintain vehicle speed, car begins to stumble badly. When car back to level ground and at set speed, no more stumble.