I know torqueflite's really good but I have never studied into torque converters enough to diagnose one but what your describing must be a converter issue because nothing in the trans. will cause this.
That's what it is, as I have seen it a few different times.
Time for a new converter as well as fluid and cooler lines flush at the very least.
If it has chunks in the pan, then it's teardown for inspection and cleaning.