If you search the yahoo group for Mediator, you will find lots of info relating to this problem.
The main one is that the graphics card isn`t fully home in the mediator. There are suggestions on how to fix this, including using washers to raise the height of the mediator so the card fits in properly.
It worked for me (until I ruined my miigy, see the thread "What have I done to my A1200 ? Aaargh!"