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1999 ford e150 van

I have a ford e150 4.2 6 cylinder. It has 98k and has been well taken care of. It burn no oil between changes. Last year I had a missat low rpms. Kinda like a bucking once in a while. My brother said it was the exhaust part of the manifold. He took it off and cleaned it good. It fixed the problem for one year.It has started up all of the sudden with out warning again. He cleaned the manifold again and included new plugs and wires and put a new egr valve on it. No difference. Under load it runs like a top. Just as soon as you let off the gas and bring in a slight bit of throttle it bucks, or misses. Put the gas pedal down and it goes away. There has been no check engine light on at all. Please help if you have any ideas. Joe

Could be a small leak in the intake manifold leaning out the mixture when the throttle is at minimum.