Hi
Welcome to Just Answer
The problem you have sounds to be an oil quality problem, the viscosity changes when it gets hot and it also expands, and the lower viscosity is causing it to give less drive when in the torque converter. When you get above a certain speed, the overdrife comes into play, this means that the gearbox is semi-locked and by-passes the torque converter.
You will need to do an oil and pan filter change on the gearbox, the pan filter is located in the sump, which will need to be removed, there will also be some magnets, check there is no large pieces of swarf attached to them and give them a clean, they may be coated with a fine metalic paste, a small ammount of this is normal.
Hope this helps
Regards
Trev
UK Car Mechanic
NVQ 3, Modern Apprenticeship, MOT Tester, 14 years experience working on all makes and models