2004 Chevy Monte Carlo Flickering headlights and interior l

My 2004 Chevrolet Monte Carlo SS Supercharged has developed a small problem. When the car is at idle (in park or in gear) the headlights and interior lights flicker slightly. But, as soon as you accelerate or simply give it gas and bring the engine RPMs up passed 1000, the problem goes away. I had the alternator tested and was told that was NOT the problem. But, my car has a 4 month old Optima Red Top battery with 800 CCA and according to the test is only cranking at 570 CCA. What's going on?