It's either coming out of a breather or leaking from a seal somewhere. Check the tailshaft seal and also stick your hand up as far as you can shove it on top of the transmission. I doubt that it is coming out of the filler, it would be smoking like a motherfucker and possibly be on fire. Keep in mind that sustained higher rpms will bring excessive heat to the fluid, making it easier for it to go places quickly (ie past seals). Sometimes, automatic transmission leaks are difficult to find, but if you clean the transmission off and then drive it to replicate the symptom, you'll find it.

Also, when you check the fluid next time, smell it. If it smells like a forest fire, you've got some other problems. Also, look at the color.

If you end up keeping the automatic, I would suggest bypassing the factory cooler in the radiator and going with a high quality bar and plate cooler mounted in front of the transmission. Remember to only use rubber lines that are specifically for ATF. Fuel lines will expand and swell.


Sorry for the novel, I used to be a transmission guy.