Thanks for your replies Akira and Hewitson.
Unfortunetaly I do not have a multimeter or a working C64 for testing. The reason why I beliveved it to be the PLA was the symptoms described on Ray Carlsens site
. But he also says that this problem can be caused by a lot of different IC's, like i.e. the character ROM.
Here's Retro-Donald who produces the PLA replacement.
Might just have to get a working c64 too, but there's nothing as satisfying as fixing something yourself :-) Hmm.. not sure what do do next :-/