It could be a bad VIC-II chip, do you have another 64 you can swap chips with?
Jay
@BibBee99 and AtariGeezer.
Thanks a lot for your help! Yes I have a spare C64 but that one is working and I am afraid to mess that one up. It is however not the cable nor the poweradaptor as my first step was to try the two C64 with the same setup/poweradoptor/RF-cabel.
But I will try and look at the pages and see what can be done. Thanks!