Like the other guy posted earlier, check inlet and outlet temps with like a temp gun rented from an auto parts store differnece should be 10-15 degrees from top to bottom. If thats good its your thermostat. Sounds like its not opening or is stuck open alrdy. Your symtoms are changing and its the only part that changes. But I went with an OEM radiator and the new one that came had more cooling lines so it was "updated" as far as I can figure. I am pretty sure my 8 runs way hotter than your 6, although the radiator may be different there also. I dont think youre tranny could be overheating it but if its slipping who knows.