It sounds as if the ignition timing is too far advanced and that you are getting pre-ignition - essentially hot spots within the combustion chamber are doing the job of the spark plugs. Switching off the ignition therefore has no immediate effect.
The juddering you referr to appears to confirm the problem. When the engine does this it's actually trying to reverse its rotation. The final whir happens when the engine goes into reverse rotation and is allowed to expire.
Having an engine's ignition timing too far advanced can seriously damage pistons and cylinder heads so I recommend that you check the timing accurately with a strobe. If the engine has been tuned, consider using a higher octane fuel or use an octane additive. aside from early Ford crossflow engines, the combustion chamber is in the piston, not the head so skimming such a head doesn't alter the compression ratio.
I'd also recommend that you check the cylinder compressions as it's possible that the head has not been converted for use with lead-free petrol. Erosion of the valve seats and/or valves may be the cause of your pre-ignition problem.
Hope that this helps and good luck!
