To my way of thinking you can do one of two things, either add an aftermarket trans cooler or do frequent transmission fluid and filter changes, kinda depends on your driving style and more importantly if you have extreme elevation changes. I live in the low country of SC now where it'* flat for 95% of the 151,000 miles that my wife and I have put on the Bonneville. Except for a couple of excursions into the blue ridge mtns it'* been a flat land car. From what I've read you can expect about 50,000 miles out of transmission fluid at a constant 195 degree operating temperature. Since you can only replace 55% of the transmission fluid in a 4t65-e with a pan drop, I generally change the fluid and filter every 30,000 miles or so, and if I'm feeling particulary trans paranoid I'll pump out about 6 quarts thru the dipstick tube between pan drops. This service schedule has worked so far, I've used Dex VI since it became available I think in 2006. I did install the trans-go shift kit at 140,000 miles to address the hard shift problem, worked like a charm. So at 151,000 miles on my 4t65-e maybe I'm living on borrowed time, but it'* performing like a trooper for now. I had mentioned elevation change earlier for this reason, back in 2007 I was visiting my home state of Wa. and had a rental GP that had trans temp in the DIC which my 2004 Bonneville doesn't. This was in July and from Seattle, basically at sea level, to the summit of Stevens Pass at 4000' elevation, the trans temp climbed to 255, at that temp the fluid is only good for about another 5000 miles. Whether or not you add a cooler, next time you do a pan drop fluid and filter change, throw out that little rectangular magnet and use 2 of those large donut shaped magnets, the little rectangular magnet was totally inadequate, there was a tsb about it.
2004 Bonne SLE
1971 Chevelle SS