Unlikely that new plugs are the fault. There are advocates of 6000 ohms resistance being significanat and others who say if it fires well on the screen under pressure testing it should be good. A defectivee plug is not likley to clear up at 1200 rpm unless it was fouled, and then more likely to need more rpm to burn off.
The carb may have problems at idle and low speed and then engine runs good when it is on main metering system above 1200.
If you start engine from cold and it fires right up it is likely that ignition system is good. You can start from cold,run 20 sec with it missing, then shut down and check which if any cylinder is cold or if any spark plug is cold, ie not firing.