@Frederic Sorry for the late response.
I faced the heating problem on my second drive itself. I fixed the extra cooling fan and did not face heating issue for long time even in summer. But rarely I get the same issue even if the cooling fan is on. However I think the cooling fan will solve the issue.
1) Pajero model (3.5 or 3.8) - 3.5
2) KMs done so far - 170000
3) Has the transmission oil already been replaced ? If so when ? - Yes, Aug 2020
4) Do you drive in tiptronic or normal “D” ? Most of the time I drive in 'D', rarely I use triptronic.
5) Tyre pressure when off-roading? - 14psi