^In my experience with the OEM HIDs, the Morimotos and recently the Osrams a flickering bulb can be either the bulb or the ballast. One of my HID Morimoto fogs also had a similar symptom when the ballast went out.
To check for sure if it is the bulb or the ballast about to go out, I would switch the bulbs position. If the flickering stays on the same side, it is probably the ballast. If the flickering bulb stays consistent regardless of where you move it, then it is the bulb.