Aye Spartacian,
Make sure the main earth strap from the battery to the frame and the frame to the engine is made from a good heavy guage copper wire, with crimped, soldered and heat shrunk terminals attached. That the frame and engine cases beneath the terminals are clean bare metal, with stainless steel or clean steel fixings, with either serrated washers or spring washers that "bite" into the cleaned metal and keep the fasteners tight. Finally, once tightened, spary librally with ACF50 and cover with a good "dod" of petroleum jelly.
New batteries etc. will not cure the problems if the main earth is suspect.
My apologies if this is telling my Grandmother how to suck eggs!
Good health to you and the Mighty Midget, Bill