had exact same problem with mine, turned out all it needed was transmission fluid topping up. Checking level with engine off will show its normal on the dipstick, you need to have the engine running and engage drive a few times so the fluid goes into the clutch packs etc. Then check the level using the stupidly long dipstick next to the battery. Took about half a litre with mine and then it worked perfectly.
Andrew, March 2011