well, i have the exactly same problem with my os 160fx, it keeps burning plugs every 2-3 flights (days of flying) and i only use os #8 plugs or os A3 when i run out of #8s..

this is just a thought, i have no idea if its correct, but it might be corrosion??? after 1 week of storage.. because i fly 3d at ground level, i need my engines to be 100% eficient and safely, and i never had a single problem with my engine when ive benn flying or i just replaced the plug... but when i go back home and come next weekend, my engine is so bad that it wont even start ot hold idle and the problem goes away with a new plug... maybe that i fly in peru (very humid weather) and i fly close to the ocean (salt contributes in corosion) my plugs get corroded when i go back home..

besides that my engine is one og the most trasted engines ive ever had, never had a dead stick ecept on the 1st flight of the day, and when i have a dead stick, i put a new plug and it becomes the most trusty engine ever...