My 2 cents....
A xenon/hid bulb works by passing electricity through ionised xenon. The arc that is created generates the light that you see. The stability of that arc is subject to the stability of the electrical input which is why a xenon/HID has a "ballast" which is a circuit that creates and maintains a very specific electrical current for the arcs.
If you have visible flicker then either the arc is unstable which can be a result of many things. It could just be a badly made bulb where the cathode electrode have been improperly processed with an insufficient thoria which in turn causes the electrode to be inefficient. OR, it could be an unstable power supply causing ripple. Equally it could be as simple as dirty electrical connections, SO...
1) Disconnect all the connectors to the bulbs and ballasts and clean them with electrical contact cleaner.
2) If that didn't work, replace the bulbs with new ones.
3) If that didn't work, replace the ballasts with new ones.
Probably a very first port of call though might be your overall electrical health in the car. I.e, is your battery healthy? Get 2 x tubes of "batt aid tablets", add 4 tablets per cell, top up with tap water and buy a trickle charger, remove the battery from the car and charge it.
See if that cures it.
Batt Aid Tablets