When it comes to ground wires, it isn't sufficient to just make sure they're tight. You need to make absolutely sure that the surfaces are bare metal where the wire is bolted down. Even if you have a ground wire from the factory that has never been removed, it can still have light, white corrosion that will prevent it from grounding properly.
I always use 120 grit sandpaper to clean the surfaces and the ground connector as well. I then apply a liberal amount of electrical connector grease to all surfaces.
Good luck on getting it figured out. If all else fails, have the battery tested for amps and deep draw ability.
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