It may be the switch, loose wire, or whatnot, but if you count all of the easy stuff out, then it is probably a control issue. If you want to bring your car into the 21st century, then I would recommend Jon Haas' headlamp controller. Here is the link:
Pantera Electronics Headlight Control Unit
Not cheap, but definitely a worthwhile investment; with many happy customers. Enjoy!
Mark
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