Paladin he already said that he tried different monitors and that they gave the same problem.
You still didn't say if you are using rivafb or not. (rivafb is a framebuffer driver for the console for nvidia cards but it is very, very buggy and can cause strange problems like this.)
If that isn't the case it might be your videocard, but that also is a bit unlikely.
Something else that can cause problems like is magnetic influence from "bad" (unshielded) pc speakers. Perhaps that's what you are seeing.