A4 Automatic gearbox

After 9 years of trouble-free motoring in my A4 1.8SE, the auto box has started playing up. It is OK for the first mile or so, but then fails to drop out of drive as I come to a halt. Setting off presents a similar problem - it stalls the engine by going into "solid" drive. I haven't worked out how long it needs to stand before recovering - probably around 3 hours.
Is the 4-speed auto box "sealed-for-life" maintenace-free, or can anything be done? Is it the torque converter? Do the hydraulics have filters? Is it controlled by an ECU? Any suggestions?