...could also be a marginal battery...
When the 750 first appeared (up until about 50% of the K0 production) the voltage regulators were an ON-50%-OFF type design, which pulsed the original sealed-beam headlights after the battery was about 12-15 months old. In 1970 the regulator was modified to act much more slowly and a different value of resistor was used on the back of it (that's the green thing behind the regulator).
If the resistor is broken (which some have endured from rough handling along the way) or open-circuit then the voltage will 'toggle' with load changes, by about 0.8v to 1.0 volt inside the headlight bucket's wiring. If the battery is losing efficiency now (or has high internal impedance, like Bikemaster or Challenger batteries, among other Chinese offerings) then it will make this fluctuation worse.
To help reduce this, I suggest running an additional Green 18 AWG wire from the electrical panel under the left cover up into the headlight, and tie it in at both ends. I do this with the Black wire, too. TwoTired (above) has it right: the connectors are aged and less than...uh...perfect now.
