One thing mentioned on the Ashcroft site is a sticking problem after changing the oil and filter, but it occurs when cold rather than hot:
I have a 4 speed ZF Auto and it sticks in first gear in the morning or when cold and is reluctant to change up from 1st gear. Once the gearbox has warmed up it shifts normally. Why?
We have noticed many customers reporting this fault starting a few days after an oil and filter change and after researching this using our experience and the expertise of others in the trade we have come to the conclusion that as ATF oil is a detergent type oil, when the oil is changed this will dislodge and stir up settled debris in the autobox which in turn jams the governor valve. In light of this although it sounds controversial it is debateable if changing the oil and filter is a good idea, some say if the box it going to fail it will fail if the oil is good or bad and their advice is leave it alone. I am still undecided but the truth is I get a lot of calls from customers thinking they are looking after the autobox by servicing it and a few days later this fault occurs.
I've also heard that regular oil/filter changes are ok
if the box has been well maintained in the past, has not overheated or been abused.
Problems start when someone buys a second hand box which has not been well serviced (or at all) and may have gotten too hot; they change the oil and filter and the ATF loosens and stirs up old residues which then clog the filter or the governor.
But as
@Saint.V8 says if it's sticking when hot it might be something else such as inadequate oil box cooling delaying up-changes. As above, I'd change the oil/filter unless you know it was done recently i.e. last 30K miles.
