After mounting my ESC on the outside, as shown below, I no longer had any thermal shutdowns. I also strengthened the tail by wrapping areas that were either weak or would twist with some Nylon reinforced strapping tape. The simple modification made a significant improvement to the tail wobble.

I have had 4 or 5 successful flights after the mods, with a CG of 30mm behind the aileron servo leads, so I am now ready to replace the lead weights with some electronics.