i would try to run just the d3100.
un hook the diehards
if you are running at 14.8v, then your resting volts should be higher13.5-13.8 on the d3100.
you should check each connection front to back, solder if possible.
i am going to presume that you have the big three done from when you put in your alt.
bigger pully on an irragi alt doesn't sound right, don't they have a hard time at idle already, bigger pully would make it turn even slower. <i don't own one or know anything about irragi>
and like was already said by someone else go wang on it and check volt drop at amp them at d3100 then at front batts the diehards then at alt terminal and post back.
and is this drop happening at will with hard wanging or only over long periods of hard wanging ?
and do you ever dive without wide open wanging to recharge?
and when you drop to 11 volts how long will it take to recover ?
enough questions
good luck, you will get it