RCIGN, ignition timing follows crank speed in theory....As you know, the timing on our engines are advanced and retarded by electrical means these days, and not mechanically as in years past. It's possible that the ignition isn't responding to transient changes in prop load/rpm changes at low rpm, or when a transient load is applied to the prop, such as a rapid stop of the aircraft, or shutting down the throttle. Perhaps the response of the ignition is dampened too much, and won't adjust to rapid RPM changes at low speed???

I'm just speculating here, as I can see that everyone seems to have exausted the fuel possibilities. Never hurts to try something different, regardless of how remote the possibility might be given that everyone seems stymied by this problem.