Well... I found something that did work for me... and I have the wifi continously working for more than 48 h now.
(to be done as admin)
1. Turn Off wifi
2. Open terminal
3. Type "sudo open /Library/Keychains/System.Keychains"
4. Search and delete the network passwords
5. Close the window
6. Reboot the iMac
7. Turn on Wifi and put your passwords again for your network
Hope that helps for you too
In Keychain Access, I removed all entries with 'Kind' 'Airport', like you advised me to do. Since then I don't lose the Internet connection anymore, everything works like it should. Of course I'm glad the problem is over, but I don't really understand what caused it. Is this a structural problem? Anyway, I'm going to enjoy this beautiful machine now, and forget about the initial difficulties.