How much time on the engine? Some engines need a gallon of fuel run through them before you can get a reliable idle. Others do well on the first tank. A different plug and the tach tricks are good ideas, but also remember that when the airplane is in the air, the airflow tends to help the engine continue to turn over, so even if it has problems on the ground it may not in the air.
Also, how low an RPM are you trying to run at? What prop and fuel?