Normally flickering lights means worn out brushes in the voltage regulator in the alternator. However, if you are actually pulling 1200 watts for an extended period, your alternator may have overheated.
I see the voltage regulator only for the Valeo alternator from my suppler. Apparently, if you have the Bosch alternator on your car, repair parts are not available. I think that either alternator has a voltage regulator in the back of it. I would pull the voltage regulator out of the back of the alternator and see what the brushes look like.