I looked up the TSB on this at work today. First of all GM says it is perfectly normal to have you lights flicker at night
"the customer should be assured intermintent flickering of exterior and interior lights is a normal occurrence"
With a GM scan tool you can turn on the air pump while the lights are on to make the car do it, to "demonstrate it for the customer".
It said you can try to replace the alternator with the specified one, which you claim you did. But this is not a guaranteed fix. THe problem is caused by the air pump (only CA and East coat emissioned vehicles have) turning on while the lights or blower fan are on causing a large voltage draw, which spikes the voltage regulator (int the alt) and it then resets its self.
So basically deal with it, or sell the car
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