[jhv48]

First of all, vacuum secondaries will not open when revving the engine under no load, so don't worry about that.

Usually a miss at that speed under load is a spark plug failure. That's the easiest place to start. Pull all 8 and read them. Report back with pics or descriptions of what you find. May be a lean condition or a spark delivery problem, but the plugs are the first culprit I would suspect before going any deeper.