my max temp driven hard in the mountains is around 167 and never ever touches 200 as it used to earlier before I put the Hayden 679.I failed to mention, I installed a Tru Cool Tranny cooler about 4 months after the transmission was replaced. My gauge reads 140-160 normal highway driving in southern California weather. Can climb to 200-210 if driving up mountain roads but that is rare (was common before the trans cooler was installed).
The cooler is inline, after the radiator, trans temp sensor located in the line out from the transmission. The cooler has been very effective in keeping the fluid cool so I don't believe fluid breakdown is the problem.