Unlike traditional light bulbs, high intensity discharge, or HID, bulbs emit light from a filament rather than from an arc discharge between two electrodes sealed in an airtight capsule of gas. HID lights are very bright but can be prone to flickering for a variety of reasons.
HID lights are high-output, low-resistance devices. To operate correctly, they require ballasts to control their current and voltage. If your vehicle is not designed for HID lights, the resistance of its electrical system may result in fluctuating power being sent to the lights, causing them to flicker.
Daylight Running Lights
If your vehicle has daytime running lights, only 20 watts or so of power will be sent to the lights rather than the 35 watts required for them to operate correctly. This, too, will cause HID lights to flicker. You can solve this problem by disabling daytime running lights, which isn't difficult.
If a vehicle's headlights are running on a 10 or 15 ampere fuse, the amount of power delivered to HID lights may be insufficient to prevent them from flickering. Replacing the fuse with a 20-amp version will reduce the resistance and increase the amount of power supplied to the headlights without overloading the wiring.