Hei!

I noticed HEAD, TAILLIGHT came up on display one day when driving. Went out, left side was blind. Switched lights off and on again, both lamps came on, worked for a few minutes and then left failed again. That was a week ago. Since then I've used fogs. But when I tried sometimes, both HID's work fine. Never left them on long enough to fail again. The same day I found this fault, I saw that if I switched off and on really quickly, then the left wouldn't light up. Reflections from a car before indicated that it TRIED to fire, I saw it flashing lightly, but then shut off. If I left an interval between switching, it fired up and stayed lit for a while.
So, what's failing? Ballast pack? bad connection? Maybe overheating issue? Ideas?  
Bulbs are new (from china) changed them less than 2 months ago.
Any help appreciated  

I don't believe it's the bulbs, they are new. And when it lights up, it burns bright and color is the same with the other. Swapping ballasts of course is reasonable anyway ... But I'll do it when I have few more ideas...

Oh crap Ok, thanks, I think I have a ballast pack lying around somewhere in garage... I installed one on my bike a few years back. As they come in pairs, I have one left over. Not Opel of course, but I can't see the difference really ... We'll I'll keep an eye on it and if it acts up again I'll swap. Thanks again for the intel. 

I have a MIG one here...MUCH cheaper than Vaux! lol