Hitting the track, but which car to take?

I have had this transmission for 3 1/2 years and the dipstick is correct. I've filled it before and never had an issue. I have a transmission temperature gauge and it rarely gets more that 185 degrees. This is a new thing that started on some of my last drives of last year and began again after three or four drives this year. It seems to be pressure related as it happens mostly when I manually shift or run extremely high speeds. It appears to be leaking from the vent tube. Also, on Art Carr's advice I removed my fin and tube cooler and installed a stacked plate cooler that works perfectly. Not temp related as I first thought this spring.