Random hesitation

My truck (1996 chevy K1500 4wd 4.3 v6)has a engine hesitation at normal speed and taking off (intermetiatly). It jerks and bucks violently for a
few seconds and then returns to normal. I have put it in neutral when it was doing this and the engine revs normally. Seems to do it under load. I changed the fuel filter.Any help or ideas would be great.
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