Hooken

Thanks for the extra detail

Your problem may be double oil. These motors do not add extra oil by themselves, you have to do it manually for the break in period. I assume you are now refilling with raw fuel to start to clear out this extra oil?

Starting fine is <U>not</U> an indication of a sufficient battery. To properly test a battery you need to measure its cranking amps by loading it up. A volt meter after it has rested for 24 hours is another good indication. What test did you do?

Your o2 readings would be causing loss of RPM but at least it is due to rich running, not lean!

I don't think you have a plug problem. I'd run it for a few hours after break in without extra oil and see if it clears up.