What state is the atf fluid?
The kzte box cools via the lower part of the rad (well, technically it uses the rad to heat the transmission quicker) and there is a tendency for the top part of the rad which contains engine coolant and is pressured, to force its way into the atf section if the rad deteriorates. A symptom can be erratic gear changing. If you take a look on the dipstick, ideally the fluid will be a nice dark red but probably brown - what you dont want to see is strawberry milkshake! Also, have a sniff, burnt toast smell (yep, really) is a sign of burnt clutchs in box.
A radiator swap is a good piece of preventative maintenance, somewhere around the 100-120k mark.
To prevent the mixing issue, you could run the gearbox through an aftermarket cooler rather than the radiator, the downside to this though is that the radiator does warm the atf up in cold weather.