Also I wanted to add:
What everyone has said is quite valid!
Super Tigers like lower nito fuels, I have one that I run 0 nitro and it runs quite well.
Problem is with the lower nitro fuels you get a lot less power, and it seems that prop you are running and the fact that you are flying 3 d would suggest that you need a lot of power to hold the plane up. Most of the guys in my club flying 3 d are using four strokes or gassers. Myself I do not fly much 3 d and tend to fly faster airplanes. Always overpowered, but very light weight, and low wing loading pattern and pylon type of stuff with some hotliners and alike thrown in. In your case you are pushing the envelope in a different way and with a low pitch high diameter prop you are spinning you engine up to it's max limit a lot, somethings gonnna give. Not being critical because I have burned out and blow apart more engines, and motors than I care to admit. The guys also have pointed out that cooking the plugs can be either to hot of a plug or to hot of a fuel, both of these can be issues. Cooking the rings tells me that you also have an issue of the cylinder being to hot, and not sinking the heat away from the rings as well as it should. Now we throw into the mix that you living in Texas where it get hotter than I care to have to endure, perhaps that is why I am obsessed with the cooling thing...lol

I sympathize with your problem, just keep in my that with so many variables it is hard to say from a distance exactly what is wrong. But you at least have some suggestions of things to try now that you may not have thought of. Now you have to try some things and see what the results are. I would also suggest you get a remote sensing temperature gauge, it will tell you a lot about you running setup as far as temp and alike they are cheap and everyone should have one in their flight box. Keep in mind you may have to run a bit richer than normal as you typically are flying at high throttle settings and the oil in the mix is not just for lube but cooling as well. It could be that your plane/engine/prop combo need to be addressed, due to the type and style of flying you are doing. Warbirds and a lot of scale airplanes (inhale sharply) at 3 d, deltas don't fly pylon that well, and high wings don't do pattern that well. I did not notice what you said you were flying you may well have mentioned the plane and I missed it, and that may not be an issue at all.

Just keep in mind you are pushing hard and to get the max out of the engine you may have to make some adjustment to keep it from breaking as often.
"She can't take much more of this Captin" Scotty...