when filling an automatic gearbox, after putting in correct type/amount of auto trans fluid, you have to start the car and keeping the brakes ON, move the gear lever though the gears to allow the fluid to settle thought out g/box, or you will find the gears will "slip". when this happened to me, the gearbox sump seal was leaking, replaced this, and put 5lts of fluid in, went though the gears, no problems since. about £25-00 for rubber seal and £6-50 per litre of fluid. all from motor factors, NOT main dealer.
hope it helps
November 2010