If I remember correctly (it's been many years since I had a 140) both high and low beams are powered by the same fuse---the one really stupid safety flaw of the otherwise superbly thought out 140---a single short to ground, and you are in the dark at 70 MPH........

It sounds to me like your problem is with the high/low beam relay. There is a clever little mechanism inside that flips back and forth between two sets of contacts. With much use it can become unreliable. Another possibility (less likely) is that the contacts are burned on the low beam side. Sometimes you can learn something by opening the relay, but it is better to get a wiring diagram and diagnose it properly (or test by substitution if you have a spare lying around) and replace the relay if it is bad.