pajero auto 5 speed ,2006 np

Its simply the torque converter locking up I would say, wifes jack does the same.
Cheers Pete

Most modern auto transmissions have a lockup torque converter that may operate in 2 or more gears.
If you increase load it is common for the ECU to instruct the auto to unlock the torque converter.
This is almost certainly what you are experiencing.
Regards Philip A

Keith
Because there is either, no slip or slip, happening in the torque converter it will mean an engine speed change so you notice it as a slight rev change.
The lockup clutch is quite small and cannot take much torque so when there is anything other than very free running it will drop out of lockup and go to slip in 5th or slip in 4th.

Possibly won't happen in 3rd because there is more torque possible in the system in lower gears and it won't go the lockup then. no point.

Any time slip is happening in the torque converter some of the engine power is being converted to heat in the oil/fluid and so a cooler is often required when towing to cater for the extra heat developed in the torque converter and fluid.

Definitely not an overdrive on 5th. You only have 5 ratios selectable + reverse.

Ross M