For the last 15 years, I take care of a building that uses MH lights. 99% of the time, when the bulb is bad, it does exactly what you're talking about. One time, I did have to replace the ballast. When the ballast was bad, it stunk like shit and there was gel running out of it. I've also had brand new MH bulbs not work properly, and the only cure was another bulb.
I don't use MH in my grow room, I only use CFL's and HPS, not that this has anything to do with your problem.