Hi Christian S,
This could be a problem with the motherboard of the computer as even if you tried to replace memory the problem persists. Replacing the motherboard of computer will fix the problem.
In case the system has active warranty, please private message the system service tag, contact number and registered owner’s name. I would be glad to arrange for a replacement.
To private message, click my user name and click start conversation.
Check warranty status: http://dell.to/zEE2Cm
In case of expired warranty scenario, we would not be able to extend warranty entitled services.
If the warranty on the computer has expired, below link would help you in contacting expired warranty services (this is for US based customers):
http://dell.to/4LOBWi
Awaiting your response.