Rising trans temps 4t65E

So My turbo grand prix with a fully built 4t65e is getting very hot. It got up to 260*f when i looked at my scan gauage . It has a aftermarket trans cooler but its not mounted in the greatest location. Anyways once it got that hot it wouldnt shift any more and felt like it was slipping slightly. No matter what you do it just keeps climbing no matter how you drive it. You can drive it for 15 mins while the trans is still cool and it shifts perfect and pulls hard. But then it starts to climb. Any one have any ideas? Level is up on the fluid and i have dropped the pan 2 times now and changed the fluid and filter and no diffrence.

Almost seems internal, like slipping or torque converter issue if the fluid will not stop rising that high no matter what.

Or the off chance the sensor is going bad. Even with a okay sized cooler the fluid temps should not just start going through the roof during normal driving.

Is there anything i can do or data log to diagnose what the problem is.

What ATF are you using ???

When I started using Amsoil ATF and Mobil-1 ATF, the trans was much happier and not as hot in the summer.

Dexron VI can go to 250 without scorching. Any other if you get to 230/240 you should change the fluid. As in chemical-less flush. Not just a drop and drain.
You need to monitor TCC to see if you are getting lockup. There is pretty much only one thing in an auto that creates the head. That's the Torque Converter. Best data stream is Torque Converter slip. Most TCC disables are the brake switch not working. Oh and most also need a POSITIVE signal with brake off. 0V when brake is applied.

Change to Dexron 6 and mount that cooler in a better location. Post tune and log file?

Amsoil ATF is much better than any other Dexron VI compatible ATF. Amsoil ATF is dex VI approved.