Changing the lockup tables is more personal preference than anything but most guys turn it off completely in 3rd and set it up in the 50mph range in 4th, but its up to you. That doesnt sound like your problem and I know what you are referring to. Do you have a scan while this is happening? What does timing and fuel do when this happens? It is more or a torque management issue and power loss from what it sounds like than a trans issue.