gearbox problems
As the ML gets older a number of minor issues with the auto box seem to be happening. A good few of these can be cured by replacing the autobox fluid. In the normal service schedule the gearbox is checked for level, and that's all. There is a drain plug, but it will only allow about 2 litres to drain from the later series 163 gearboxes. The torque converter cannot be drained. So, drain into a clean container, measure the amount that comes out and put the same amount of synthetic ATF back in. If the fluid that comes out is brown rather than pink, run it for a few miles and then do it again. And again. Then at every service take out the same amount and replace it with new fluid. You'll find the smoothness of the gearbox restored and the hiccups and hesitations will disappear.
I know of three MLs that were sold for next to nothing because of gearbox problems and this treatment cured them.
To avoid this repetition of manually changing fluid and have the new fluid contaminated with the old; it would be better to select a specialist who has equipment that will purge the torque convertor and gearbox oil cooler circuit and replenish with a totally fresh charge of fluid, remaining uncontaminated and hence last longer. A good point raised.
It's normally a circlip that causes the problem with gear changes, oil replacement will no prevent this, as a service item I agree this is imperative.