I had the same intermittent problem too Matt, can you swap the ballasts from side to side and see if the problem persists? Thats what I did and was able to identify it as a ballast problem.
I'm going to change to 4300k shortly too I think, the general consensus seems that they're the best compromise between light and colour.
These aren't cheap, but seem to get good reports -
Hids4u.co.uk