Thanks for the tip, DK, and thanks for your explanation, Tacit. Luckily, the iMac is still covered by APP, so I will probably take it to the shop, if moving to a different location and using a different plug doesn't help. I wouldn't like to move it definitively, though, as it is now standing on a beautiful (fixed) desktop in my office, as you can see in the following pic. Note to the right (part of) the iMac G4 which was being replaced at that time by this monster.

http://i269.photobucket.com/albums/jj46/LesC_photos/2iMacs.jpg

There's no way out of here,
When you're in here, you're here for good (David Gilmour)