I have a PTC 3500ish lockup that isn't really locking anymore, if I turn on the lock switch and push on it the least little bit on the highway it will slip. My question is could it be anything other than the converter?
Ya single disc converters don't like locking at WOT, not for long anyway....lol I wore out 2 kevlar clutches and broke 2 forward drums before I gave my head a shake and called Dusty for a PTC 9.5 N/L. (Love it!)
One other item to check would be to see if your brake switch is adjusted properly, maybe the tcc solenoid is losing the 12 volt feed. Just a thought...