Hi,
Have you put a scan tool on it. In live data you can monitor turbine speed ( torque converter) and see which bit of the box is slipping, also it should have a rev sensor at the back of the box so can monitor that as well.
If the oil is smelling burnt then your best sending box and torque away for a rebuild.
Give jt auto transmissions. A ring for advice and rebuild cost on 01322330052