Id say bulb...one of mine did that.
Easiest way is to pop the bumper off and swap the bulbs round..if the problem is still there then it could well be the ballast.
Mine was just a bulb too...
Give your bulb that works a whirl in the other one, shouldn't need to take the bumper off though? Get your hands down there you big girl :rasp:
I've got a pair of old lamps out of mine you can try if you want, one of them is a dodgy and flickers but one of them is ok.