At least you know what your fish can stand temperature-wish. It may be time to let the cooling effect of the earth (the dirt beneath the pond water) work on cooling the pond down. Have you thought about backing the aeration of an hour at a time OR moving the diffuser into shallower water?
I really can't say what's the right way to go here, but moving the diffuser to 4 or 5 foot of water would still circulate the upper half and keep the D02 up, but let the bottom waters alone. Just a thought with no experience to back it up.
