Sounds like a solenoid problem to me and too tight of a converter.
I had the same issue with a solenoid I could hear it click in the pits then once I did my burnout it would not hold and it is too loud to hear it click or not so I changed the solenoid and that issue was fixed. But I wasn't having the slow 60' that is why I think the converter is too tight.
How big is it suppose to be? If you unplug the 2-tep how high will it go under full throttle?