I have a 09 g37 coupe and the low beam/highbeams both come on when I turn the lights on but one side or sometimes both flicker and shut off. I can turn the headlights off and on again and they will come back on for a short period of time but one of them always flickers and shuts off eventually. What could this be? Ballast, bulb? The drls and the side marker lights all work fine

 

As you said, i would start with the ballast and bulbs

 

Normally, whenever HID's flicker and go out but restart fine only to repeat the symptom again indicates that the ballast is starting to fail.

In contrast, when the HID bulb itself is nearing "end of life" the light output will become increasingly "pink" in color and go dim before eventually going completely out. And stay out.

The only potential "red flag" is that you indicate both sides exhibit the same symptoms. For both sides to fail at the same time... those odds are astronomical. Possible but not really probable.

Determine if one side is "worse than the other" and start there. Check wiring connections and the fuses in the IPDM/er. If all checks good and the issue persists then you can look into replacing the ballast (and bulbs).