I have a 1991 Honda Accord LX (250,000 mi) it has a rough running zone between 1500 - 2000 rpms. Worst after car has warmed up. The distributor, cap, rotor, ign. Wires, spark plugs have been replaced. No loss of power or missing under load. It accelerates normal. A little rough idle when cold, but expect it to with that many miles. It seems to run rough at times at normal highway steady speeds.
Anyone with ideas?

Replacing igniter and coil should solve the problem.

I was under the impression that the coil was in the distributor. What is the ignitor and it's purpose?

Both the igniter and coil are in the distributor. Igniter are electronic points for opening and closing the primary voltage in the coil.

I had replaced the complete distributor with another known good one.

Okay.