what would cause surging or chugging in a 1998 chevy s10 4 cyl 2.2 liter?

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this pickup surges or chugs when just trying to maintain speed.it never misfires or chugs at all when putting a load on the engine such as going uphill,nor does it do it when coasting downhill.It acts completely normal when taking off form a stop or building speed to get to the speed limit