I had a similar problem with my truck a few years ago. The solution in my case was a new fuel line from the fuel pump to the carb. It was releasing some small rust particles that would keep the fuel flow to the carb restricted, so that when driving with a load on the engine it would starve and start bucking. It was sometimes an intermittent problem, as the small particles would eventually pass through the needle valve in the carb and allow it to act normally. It seemed to act up at the worst times, like when attempting to merge with traffic.
