If after it leads me to believe something went wrong with the j mod procedure.
If you are confident all the holes in the separator plate were drill in the right location, and to the right size. And all the check balls are there, and in the right place, the separator plate(s) may be warping upon temp. increase causing too high throttle pressure.
I'm inclined to suggest re-torquing the valve body with a known calibrated torque wrench in the proper sequence.
Another is a sticking/leaking accumulator piston. IIRC, you may want to try re-installing the bottom spring.
That's a simple thing to do, hence why I make these suggestions first.
Other things that come to mind not j mod related:
Sticking 1-2 shift valve.
Bad pressure regulator valve/bore. Ever hear any buzzing sound when it's hot?
Sticking/stuck/gummed up clutch drum check ball.
Any runaway rpm's during the shift?
Checking transmission pressures with gauges, and duplicating the complaint, might give clues.
1995 Thunderbird LX 4.6l 97 Headlamps/Hood All Red Tails Muffler/Resonator Delete
2004 Tahoe 5.3 4x4
2008 Acadia Limited AWD
...still looking for a clean, low mileage, rust free 96/97 Thunderbird 4.6