Did the vibration start after a maintenance\repair procedure? In particular any procedure that involved removal of the spark plug wires? If so check the firing order of the spark plug wires. It's easy to get #2 & 3 spark plug wires interchanged.
Also have a look inside the distributor cap for tracking (shorting) between the terminals.
Have a look at the spark plug wires at night while the engine is running to see if there is shorting between wires or shorting to the engine.
Feel the spark plugs. Are they all evenly warm after the engine has been running for awhile?
How long since the last tuneup? When was the advance last serviced? Kind of a long shot but since the engine seems to run a bit better at higher RPMs maybe the advance is stuck advanced position.
ex TRUE GRIT
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