Alcohol is the right stuff. With these scanners though, clean not only the glass but the white backer that match up on the other half along with the infeed rollers. I have seen it happen where you clean the glass and the mark is getting tracked back ...
What you are looking for is GPR 58 toner. You were close. Just make sure to buy genuine Canon stuff though... I have had to repair way too many machines were users tried aftermarket. Hope this helps!
I have seen this type of issue using wireless networking with these large format machines. Try switching to wired or usb and seeing if this solves it, if anything just for a test.
