When was the last time you replaced the fuel filter? My '85 pickup did this exact same thing on me on a Sunday evening 150 miles away from home. It would idle fine and drive at slow speeds ok, but anything involving a lot of fuel and highway speeds would cause the engine to buck. The fix was a $5 fuel filter (located above gas tank and easy to replace). If you have no idea when it was last replaced, this would be the first thing I'd be doing.