It seems like you have a few things going on impacting your alternator. 4awg wire is marginal- I upgraded to 2/0 and saw immediate improvements. Having an external regulator allows you to minimize excessive demands on alternator- your internal regulator is causing problems. Since you have 500 watts of solar, I don't understand why there would be much demand put on your alternator- the solar should be providing 100 to 150 amps per day.