Daxter, It sounds like your box is due for a rebuild. If you intend to run the current power or more, your current (stock) box is not going to cut it, Give Mike at MV Autos a call on 08 8370 0430. He will be able to tell you what you need to know.
Once your box is sorted you may like to run a manualised automatic, this means you change gears when you want to, not when the transmision computer wants to. Check out the link in my sig. (this is not for everyone)
Hope that can help in some way.