Same thing was happening with mine. Driver side bulb would just randomly just go off. I'd turn the lights off and on and they'd be working again, but would eventually just turn off again.
Swapped out my bulbs for a pair of philips and they've been good since.
Other than swapping out the ballast, you may want to try out another pair, but different brand. If you get a ballast, I'd get a new one and oem. I believe osram is oem.