I had the same problem on the R60/6 I ride, and the problem turned out to wear and tear on the wiring harness where it goes from frame up into the headlight bucket. After beating my head against the problem for awhile (and testing most of the stuff people mentioned here) I finally took it to a shop where they wanted to 1) cut off the head end of the wiring harness and splice a new one in, or 2) replace the entire wiring harness. As it turns out, they were able to fix the problem with an skilled application of electrical tape, but be aware you may embarcing on an expensive repair cycle and you'll want to test the wiring harness through the full movement of the handle bars!
Non omnes qui habent quadrigae sunt auriga. - Anon
I returned, and saw under the sun, that the race is not to the swift, nor the battle to the strong, neither yet bread to the wise, nor yet riches to men of understanding, nor yet favour to men of skill; but time and chance happeneth to them all.