oil on plugs

[lil bubba]

I have a '08 Yamaha 70 2 stroke. I was using off brand reg. no ethanol and getting oil buildup on plugs so bad it was like tar almost....I started using shell reg. ethanol with stabill and went a year with the same plugs and never cleaned . no oil buildup. shell came out with 89 octane regular ethanol free and I started using that and continued stabill and in less than 3 months plugs fouled with oil thick as tar again....drained tank and put in shell ethanol with stabill and after running ten minutes motor running like new again.....main problem was running rough when idling , after high speed jet kick in it ran fine...all 3 cylinders foul the same with the non ethanol gas...anybody else experience this...I only have a 6 gallon tank and I never use gas over 3 weeks old...