The ZF 5HP units are also used in the D2 (94-2002) Audi A8's and exhibit a similar number of issues (I have an A8 as well as the X5).

Some of the problems the Audi guys have seen:

1 - Hunting 4/5 (sometimes 3/4) - low revs with no accelleration at around the shift point, the revs will hunt up and down as if the box can't decide which gear to be in

2 - 'Shunt' - 60mph or so with a slow down, TC will drop lock and then bang back into gear.

Both usually sorted or at least reduced with a filter and oil flush (Audi also claim sealed for life). Newer models also suffered from the F-seal failure and would either only select reverse or not select it at all.

Preventative maintenance is usually better than cure, and 40-50k oil swaps seem to be the best option. Though in Europe we're a bit more spoiled for ZF shops, my local one does a oil swap using a special machine of some kind for about £150 ($220ish).