Runs rough with light throttle at 40 mph

My 1997 E350 Supervan with 7.3 PSD is acting up while cruising with a light throttle. Most noticable between 35 and 45 mph. Engine starts easy,idles perfect, and accelerates smooth, but cruising with a light throttle you feel a jerking or surging. Tha van does not have a tach, so I do not know what the RPM is doing. You do notice it a little at highway speed, but it is not as noticable. The check engine light has not come on. The vehicle has 192,000 mile on it, with no major engine work done. Still original Auto Trans.
Thank you in advance for any ideas you may have.