If, as you say, that this battery is worse than the old one, then why should it be bad earths, as the old one was connected to the same system? To me, in this case, it points to the battery itself. If someone on this forum has a similar model, perhaps they could check the Cold Starting Amps (CCA) on their battery. Your 330 Amps seems a bit low for that engine.
Those were my thoughts Doug when I suggested a full charge and volt check on battery near the beginning of this thread. I also thought the capacity/CCA a tad low but the effects of this would not show up until the battery had reached significant age. Even this battery as new should spin the engine over fine. Remember the factory M3W battery is only 30 a/h and has to turn over 1000cc pots (bigger than those on a Dodge Viper).
Still a good idea to check battery earth to chassis and flexible chassis to engine block earth strap, but if it was generally ok before fitting this new battery, in this case it's unlikely to make a difference.
