I'll bet that it always corkscrews to the left doesn't it? I'm betting that it needs a little right thrust. That is, assuming that all flying surfaces are relatively straight and that it flies ok on a straight and level course.
You can verify it if you pull straight up at full power and release all controls. If it pulls to one side consistently, that's generally a thrust issue. If it pulls left, more right thrust. If it pulls right, a little less right thrust.
Just one more possibility to check out.