I had the exact same problem, especially as the car gets warm. Typically a flickering issue is in the ballast and NOT the bulb. The D1S using an internal ignitor, which is like lighting a stove--it simply needs to ignite the ONCE, and a steady stream of power should be supplied by the ballast. What I'm trying to say is, once the bulb is ignited and lit, the problem is no longer in the ignitor.
The best way to diagnose is to swap bulbs/ballasts left-to-right to see if the problem persists.
For me, the problem started with the headlamps flickering occasionally while driving, which was almost unnoticeable...to where it would flicker for a few seconds at a time, to then flickering and completely shutting off. Turning the switch off and back on should reignite the bulb, in which it will start flickering again. This issue drove me insane for a few weeks, but in the early pre-death cycle of the ballast, it was difficult to diagnose as the issue was sporadic as it went away as I got home.
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