I want to get my two cents in here on this one. If your motor has a bad seal or crank case leaking it will have an erratic idle even with the handle all the way slow. Because it is drawing un-carborated air causing it to run lean. You have checked the things that should of fixed the motor. I had a problem like this before and I opened up the hood as the sun was setting and I noticed spark jumping along a plug wire. Wires looked good but spark jumping to ground would kill a cylinder every time, just something else to check.

dale