Why does my van buck between 35 and 40 when maintaining speed

Engine is misfiring .. Maintaining speed actually puts more load on the engine than accelerating due to the transmission is in the highest gear available at that time .. Needs a professional quality scanner connected to check for codes plus it can be used while test driving to determine which cylinder is the culprit then the 'reason' for misfire can be proven.

Just a curious addition to pushrods observation.....and he could answer this also, probably, but is it possible on those to have an issue with the transmissions 'torque converter 'lock-up' function? As pushrod also stated, scanning will reveal the true culprit!!