It might be a, sounds like a bad ground. start at the battery negative clean post and wire end, replace after applying dialectic grease. then follow the wire down to where it connects to the frame do same clean and grease. look for additional ground wires (any electrical wire that is bolted or screwed to the frame or engine).
A bad ground can work okay when sitting still then you crank the throttle create some vibration and the ground is compromised.
Black plugs are not really a problem necessarily. Clean them use compressed air. What kind of plugs are they, regular stock replacements? Is your battery older than 3 years, then replace it, have you ever charged it with a battery charger above 2 amps (like a car charger)? if so you may have fried the battery.
Coil and points may be a problem as well.
1980 CB750C DOHC
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