Gear change not working in semi automatic/M mode

[Sattani]

Hi guys, I drive 2005 E55 AMG. Can anybody pls enlighten me as to why my gears are not staying put in the relevant gears I put them in when driving in Semi automatic or Manual (M) mode. The car seems to drive absolutely normal in (D) drive mode but keeps on automatically changing gears and puts it into D when driving in semi-automatic or M mode. Obviously there is a fault somewhere! Has this happened to anyone else? Does anyone know what it can be? Could it be an electronic module failure to do with the gear box etc?? Help required urgently!!! Thanks!