hi. since the xenon gas needs to be turned into plasma state ~30.000 Volts are required, there is a transformator of 12V to 30,000. if the Voltage is under about ~9 V the light will start flickering. if it fall far low than 9V than the flicker intervals will get longer(about 5-6 a second). (actually it flickers normally but >80 Hz so you cannot feel it!)
it can be due to:
-non original power cables which causes bigger resistance.
-but i guess it is due to your car battery lost its performance and can generate low voltage.
seems like you need a new battery