Back when I had a V10, it started reaching temps that were higher than normal. As the OP, took it to an independent mechanic and we found that the tranny fluid was cooled by the radiator and a small factory external cooler. They also found that the radiator needed to be rodded. So they removed it and took it to the local rad shop and they boiled it, cleaned it and rodded it. Never had an issue after that, in fact temps very seldom moved even over Wolf Creek and Monarch Passes in Colorado pulling my 12k 5er. I have always used Scangauge to monitor temps exactly rather than trusting the in dash gauges. Just my 2 cents worth for others.