If it's the tune, lower the tcc unlock mph table for 4th gear, about 5 mph, it may be too close to the tcc lock mph value.
Any slight variation in the tps will lock and unlock the converter; if these values are too close.
the 80e does unlock when tps is around 0%, so the truck will coast freely, that is normal.
also, something else you can try, increase the min pressure for the tcc table, just multiply it by 2 for starters.
good luck,
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