I would take a look over the tune again in all areas for line pressure/shift firmness/ shift timing for each shift and make sure things are close to stock or just put completely stock tables back in. Max line pressure can cause this but you would likely notice this during normal or hard driving without going WOT. There are a few things in the trans that could cause this but isnt real common. Also if you can scan while it happens watch the pressure control solenoid commanded and actual current and see what the numbers look like so this will verify a pcm or internal problem
