Lets review:
Greg says he put a external cooler on trans and bypassed the rad trans cooler and it still does it.
The radiator is new.
There has to be something wrong with the coolant flow or air flow.
When the motor is overheated, is the coolant in rad overheated also? If not, then the water isn't flowing properly. I would start it up cold with the rad cap off and watch to see if when the thermostat opens there is a gush flow of coolant happening. If not, then take the thermostat out, temporarily, and see if overheats. If you don't see a fast flow with the thermostat out then something else is obstucting it, or the pump has some sort of problem.
If everything is piping hot, motor and rad, is the air flowing through the rad fins properly?
It seems to me with the trans unhooked, that looking and feeling, or measuring the actual temps, of how the cooling system is working should give you an idea what's wrong.