Agree that it’s certainly worth checking the fluid level (engine must be ticking over while the level is set). In addition, the transmission controller has a ‘cooling strategy’ mode which monitors the fluid temperature and if it exceeds a threshold value, downshifts the transmission and inhibits upshifts to keep the oil pump speed (& therefore fluid flow through the cooler) at maximum. Usually restricted coolant flow through the main radiator or the oil cooler are the culprits (they get bunged up with age), although occasionally it can be caused by a faulty fluid temperature sensor in the transmission wiring harness.

Phil