I would drain the fluid from the torque converter and the pan and refill with Mercon V and add a new filter. If you don't know when the last time it was changed, it's probably wayyyy overdue.
Let me ask you this, have you checked to make sure its full on fluid? What color/smell is it? Does it look brown like it's burnt, or is it still nice and red?
These are highly engineered precision vehicles, the first step in diagnosing the problem is to strike the suspected offending part sharply and repeatedly with a blunt object, then re-test.