W220 S500 HID lights flickering

[FranksMerc]

Hi, everyone..my 1999 (V-Reg) w220 S500 has developped a fault with both inner headlights ( I believe HID?)..basically the bulbs located nearest the chrome grille, both sides, which I have set up as daytime driving lights,when switched on, begin to flicker, like a neon sign that won't illuminate properly...is there a common fault I can check for, or is this a sign that both lights are defective simultaneously, please?
Thanks for any info.