If your trans cooler is attached to the radiator (push through zip tie style or screwed on somehow) without an air space between the two it will transfer heat to whichever is cooler.
I would make sure the advance and jetting is correct as previously mentioned, and make sure the fan is sufficient with a good shroud.
As a trial you could rig a spray bar in front of the radiator and spritz it when it gets hot.
If that gets the temps down enough to keep it from vapor locking you know the engine overheating causes it, as opposed to heat soak in the engine compartment.
Good luck.
