I don't know, Brad. I never had problems until this year. My old lamps start fine, but the new ones ... nope. Starts, stops, starts, stops. This can be anywhere from 1 minute to 10 minutes, in some rare cases up to 20 or 30 minutes (but very rarely, as in maybe only 2 or 3 times in six months). I have tried them on a borrowed pulse-start and also an electronic ballast and on THOSE ballasts they start perfectly. So it could be something else, sure, but, it could also be this, I wouldn't discount it just because you don't have problems. If you take a look at
http://www.ushio.com/files/Aqualite%200802.pdf it says the recommended ballast for 400W is M135, "pulse type ballast with ignitor."