I’ve come across this exact issue several times over the years. The problem is a faulty torque converter.

I would definitely clean the throttle valve since it’s quick and cheap, but I doubt it will solve the problem.

I have seen this issue too, torque converter clutch was locked on. replaced with rebuilt one with the sonnax kit already in there, And that car shifted nothing short of magic level good.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)