[HO455]

It sounds like a light throttle cruise miss fire. Steady RPM with small throttle opening is the most difficult time to ignite the air fuel mixture. There is lots of empty space in the combustion chamber that the flame front must jump across to ensure complete combustion. This is why there is a vacuum advance in the ignition system. Start the fire early so it can burn all the way across. You should check to see how much vacuum advance is in the distributor. You can do this by disconnecting the hose to the distributor (plug the open port on the carb) and pull vacuum on the distributor with a separate vacuum source, measure your timing (an adjustable timing light makes this easier), and record the number. Getting a vacuum reading at speed when the symptom is noticed will be helpful with this kind of problem. Compare the amount of advance to the vacuum reading at speed.
Ignition weaknesses will show up during this condtion. You may be able to detect the cylinder affected by inspecting the plugs. But to do this you need to shut the engine off during the misfiring and coast to a stop and pull the plugs. Pulling over with the engine running is likely to recolor the plugs for idle conditions. New plugs will help make it easier to read the plug.
Anything that causes one cylinder to have an improper air/fuel ratio can cause this symptom. What intake manifold are you using? Some aftermarket manifolds are poorly designed and can cause this type of problem. What style of plugs are installed? Changing to a longer reach plug may help. Doing the above recommended vacuum readings may indicate a bad valve seal which could be pulling exhaust gases back into the combustion chamber and leaning the mixture out. A vacuum leak into one intake runner can be detected by spraying carburetor cleaner around the intake gaskets. Some intakes have vacuum ports into individual runners these should be carefully inspected for leaks. Preferably with a hand vacuum pump with a guage to check the rate the vacuum bleeds off. Dont forget the vacuum advance pull off itself may be leaking.
Up to this point I have concentrated on lean misfiring. It is possible you getting unatomized fuel to the combustion chamber (i.e. drops of raw fuel flowing into the chamber). You may be able to see this in one or two plugs looking sooty. Looking at the floor of the intake runners may show signs of fuel puddling.
Sorry for the long post but this a difficult symptom to find the source of. For me I need to take notes and make a concerted effort to only make one change at a time to make progress on the solution.
Good luck and keep us posted.