[Affro]

Hi From NZ
I wonder whether someone could help with a puzzling situation re our Mk I 1998 Megane 70,000 kms. When coming to a stop on a slope or hill, the gearbox still in D, it seems to drop out of gear and runs back as if it is in N. If you slowly press the accelerator, at about 800 rpm it re-engages with a thud or thump and takes off as normal. It's ok on the flat, but if you "floor it" at the lights it does the same as above. Might it need oil? I thought these boxes were sealed?
Any comment would be appreciated
thanks = Affro