Sluggish A4 Transmission - any ideas?

[Juzzo]

I've got a 2000 A4 2.4 Avant with the 5 speed tiptronic autobox. When driving in standard "D" mode the car insists on changing up at the earliest possible convenience and if I drive up a hill the car will struggle in the tall gear with the revs gradually dying. If I then depress the pedal far enough to make it change down (or maintain speed!) the damn thing will change down and shoot off! It's a bloody circus just trying to keep an even speed.

Does anyone have any clues what could be causing this? I've not driven another A4 to compare but there has to be something astray.... J.