I finally switched the bulbs, as the problem became more frequent. The problem stayed on the passenger side, so not the bulb. The schematic shows something called a "combination switch" but I'm not sure how this relates to the headlamps. Might M.B.DOC be referring to the exterior lamp switch, or is the likely culprit indeed this mysterious combination switch guy? Trying a switch seems a cheaper place to start than replacing an ignition unit. Any other suggested diagnostics?