I had issues like that for a while on my B3. After much frustration and attempts at cleaning things I replaced the battery cables and alternator charging wire completely.
Got some military style terminals for the battery and some big fat lugged welding cables from a place that does RV and off grid power products.
The military style terminals are great, lots of places to attach extra wires. For the ground wire I ran a pair, one to the engine and one to the body. They connect at the battery terminal.
Post install my starter cranked noticeable faster and I haven't had any issues with starting since. Been nearly a year now.
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