I think you found the problem with the antifreeze. When you go more than a 50% mix, you start loosing cooling efficency. Anything over 70% is horrible on heat transfer. If you're in an area where the temp drops below -25F, use the stronger mix, but only in the winter. And a 50% mix, which should show about -34F on your antifreeze tester, is recommended for limiting corrosion.
