Why doesn't my van drive smoothly at a steady speed?

When was its last good tune up? Fuel pressure and flow good? Any codes?

Well, your van is almost 25 years old, so there could be any of a plethora of problems that range from a simple tune-up to a worn out engine. But I'd first have a mechanic diagnose the problem, and expect that he'll find a clogged/restricted exhaust system, likely a plugged up catalytic converter.

plugs are old/rotor ?; runs strong under acceleration/70+ mph; no remarkable codes; ~105k mi/new timing belt @ ~100k mi. Only bucks at steady speeds ~ 40-50 mph w/wout cruise why at this particular speed range?

using Cataclean. Thanks for the suggestion We'll see...