Is the tranny locking itself up, or are bypassing it with a mystery switch. Next question, when you rebuilt the tranny did you re-torque the 3-4 band upon reassemble.
Everything is torqued to spec. The only problem is that when the converter is manually locked under full thottle , it wont shift into 4th. Just talked an expert. Said my tps might need to be adjusted. Going to check it soon...