Terios Auto gearchanging


I recently tried checking the transmission fluid on my 1998 Terios automatic. This was an attempt to resolve erratic automatic gearchange problems. Sadly the fluid check was fine and as it should be on the level and colour of fluid etc. It is still changing gear almost when it likes and virtually not at all at higher speeds, and is left screaming at these speeds.
Does anyone have any suggestions before I part with god knows how much for Daihatsu to look at it.