Ignition breaking up at higher rpm

Could be as simple as bad coil wire, an open / broken plug or plug wire, or problems with cap and rotor. AND CHECK VOLTAGE to the ignition. If you can't resolve that part, "hot wire" it temporarily with a clip lead from battery right to coil +. Don't run it that way too long, just to see if it "cured" it

You say you swapped the coil? I would check and change the plugs If you have a tach disconnect it and test