[Youngstorm]

I don't know how many other grounds you have (eng to firewall, batt to body, etc.) but this as the main ground needs good contact with engine side. A rubber motor mount with steel sleeve probably doesn't ground to engine. The engine needs a good ground for alternator, spark and sensors current flow.

 

[Youngstorm]

Electricity flow (current flow or electron flow) is moved in the entire conductor by application of a voltage force. For the same size conductor, a solid wire will have less resistance than a stranded wire. The outside of the conductor is effected by dissipation of the magnetic and electrical field. So a solid core wire will be less effected by field dissipation.