I remember you have the same problem last year DJ. I have to think that it's somehow your climate since it's the same overheating with two different engines.
The fuel you came up with may be the solution, but I would wait until the pinch on the .21TM is gone first.
I ran mine yesterday and today w/outside temps 71% and temps didn't go past 210% w/ 30% O'Donnell's.
Have you tried any other fuel besides the sidewinder, or is that all you can get?