RustyRatRod said

Get a closer shot of the crankshaft parting line on the crank throw, please. It appears to be a forged crank, but lets be certain.

Yes, that's the way an out of balance engine will respond. Shake at low rpm, then get much better or disappear, then probably reappear somewhere higher up. That's sure what it sounds like. Another possibility is a broken valve spring. That could cause a skip down low and when rpm increases, compression can blow the valve back shut and make the skip disappear. I've seen it several times.

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