Just because it came with the engine doesn't mean it the best plug for your application.
The A3 and #8 are on the hot side of the OS chart, those are for lower nitro applications
I'd try a colder plug

So what does the plugs filament look like when it comes out of the engine?
Is the filament deformed smashed, or broken? That can happen when using a hot plug with high nitro and a high compression or new engine. You may be getting pre-ignition, this can smash the filament

Is it whitish and no longer shinny? that would be a sign of over heating and a lean run

I use a MC-59(medium heat) in my small .12 engines on 20% nitro
And a MC-9 (cold) in my .21 engines on 30% nitro

I've never had one of these go bad prematurely