One of my 400 w HPS has the same problem, only mine does it more often. I'm not sure it's the bulbs. probably once a month i come down at 8pm (lights on) to find it sitting there sparking but not igniting. I have 3 bulbs that i've been rotating and as soon as i put a 'new' bulb in it fires right up. The bulb i take out is set aside for the next time i need to swap. I should mention that all three of those bulbs are HTG brand bulbs. The agromax bulb that i have in my other 400 w light has never given me a lick of trouble. So i dunno if it's the bulb brand or a bad ballast or what. I'd try a different brand of bulb but HTG keeps sending me replacements!!!

dunno if that helped you at all...but thought i'd share my frustrations.
thanks
Skeet