Electrical fault

[His Dudeness]

This is a bloody nightmare problem :lol If the revs are having an affect you should probably check what voltage you're getting at the battery when the bike is idling and when you raise the revs. The voltage should increase above battery voltage when you rev it with the lights on and off. I still think you have a poor connection on a wire that's common to to your head lights, break lights and horn. Have a look at the wiring diagram and you'll see the common wires. It could be a bad power or earth that they share.