First, fix the air bubbles. You probably have a pin hole in a fuel line or some other air leak. The thing about bubbles is that they make the engine run lean. You can "tune them out" on the ground and the engine will seem to run ok even with the bubbles. But in the air things change, and the engine will lean out a more than normal. And a lean engine run can kill a plug.

I also agree that using a bigger prop might help you out a bit. While a 10x6 isn't going to hurt your engine, you will be running at a pretty high RPM, and with a new engine (you said 15 flights on the plane, that's for the engine too, right?), you'll usually have some prety high compression. If you want to run at that RPM, you might need a colder, heavy duty plug meant for high RPM operation. The OS #8 is a good general purpose sport plug, but there are better choices for high RPM running.

piper chuck's engine setting instructions are right on, but I want to add a couple of things.

First, all engines are a little differnet when it comes to the needle valve sensitivity. On some, backing off of peak rpm "2-3 clicks" is plenty. On some, you need more like a half-turn. On others, just 1 click is enough. It depends on the design of the carb, your fuel, the weather, and other factors. I have a TT .46, and it has a very broad needle range (a good thing) (at least the way I'm running it). So you might need more than a few clicks when backing off of peak rpm.

Second, if you can't pick the plane up to do a "lift test", you can also pinch the fuel line (if you can reach it). Give the fuel line a good hard pinch-and-release and listen. The pinch will block the fuel flow for a moment, causing the engine to lean out. You should hear the engine pick up RPM. If it sags or the RPM doesn't change, it's too lean already.

Remember that in the air, the engine will run at a higher RPM than on the ground. How much higher depends on the prop, airframe drag, and other factors. This increase in RPM will require more fuel. Since the mixture is set on the ground, the effect is that the engine runs leaner in the air than on the ground. Sometimes much leaner.