If you go to the Horizon site you can download the manual for the engine just in case
you didn't get one with the purchase of the plane / engine combo. I see it suggest a 12-6
for normal flying, you using a 12-7 could be a small part of the problem, but shouldn't
cause a lot of the things you are stating. I also shows a 10-6 for break in, so you might
try in between the 2 sizes to see which works best for the engine / plane combo. It may
also give a little insight into what adjustments you might need to make to make the engine
more reliable.
http://www.horizonhobby.com/pdf/EVOE10GX2-Manual-EN.pdf