It could be a lot of things, but I'll take a stab at a few. When you do the pinch test, the engine should run for about 7-8 seconds before it dies. If it's stalling too quickly, you're too lean on the bottom. The temp range doesn't seem too high, but the if the ambient temperature is very cold, then 220-230 might be very hot.
Be sure to use new fuel.
Check the top of the piston for a sandblasted look. If it's clean and has a fresh sandblasted finish, then you're getting detonation, which can take out a plug pretty quicky.
Don't run too much nitro. It can cause detonation as well, which will bury the plug.
Is the engine relatively new with good compression? A worn engine can be tough on plugs.
Any chance you can post a picture of your plugs? Sometimes the diagnosis is easier if we get to see what's happening with the plug.