The internal parts could be welded solid and not stall the engine IF the converter is operating correctly.
You have a problem in the pump or valve body.
Lock up valve with springs in the pump is a common item installed backwards by a Newbie DIY person
Is this converter a fresh rebuild or old re-used converter?
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.