Join Date: Oct 2013
Location: Chilliwack, BC, Canada
Surprised that no one has weighed in.
High and low beam are powered from the headlight fuse which is located near the battery unless someone has improved and relocated the fuses. I'd check the fuse and its connections in the fuse holder.
Both high beam and low beam derive power from the headlight fuse, as I said, and have common wiring from the ignition switch, back by the brown wire to the fuse and then forward by the blue/white wire to the dimmer switch. From the dimmer switch, the high and low beams are on separate wires to the headlight.
High is red/black and low is red/yellow.
I recommend that you download a wiring diagram and trace the circuits to become familiar. For those not used to such diagrams, it begins by seeming to be a mass of confusion but use a pointer and following a wire unravels the mess. I recommend making your own diagram by simply drawing the components in question on a blank sheet of paper. I like to place them in their general relationship as my mind sees the bike.
Once the components (such as headlight, dimmer switch, headlight fuse, are on the paper, draw in only the wires which apply to these, including the grounds. From there, the problem becomes much easier. I often copy the diagram and then add additional items as needed so that I an go back to the simpler one if needed.
Since the intermittent seems not to be addressed by the dimmer switch and since I assume that you have a US or Canadian bike which lacks a headlight off switch, I'd proceed by removing the black wiring plug cover from in front of the gauge pod. Big plastic bucket held by two Phillips screws. From there, begin tugging, wiggling, twisting wiring to see if you can cause the light(s) to either come on or go off. This should help to indicate a loose connection and indicate the location. Many other options but I'd start there.
Do you have a voltmeter and/or test light?