It could be a bulb/ballast incompatibility issue, but is more likely a bulb positioning problem. Try rotating the bulb until the
[hr]
inside the bulb is between the 10 and 2 position.
Though the problem may also be the result of just running a new bulb. When I installed 3 new bulbs a couple of months ago, one of the three would do something similar. After a few days of burn-in, it now starts constistently.
Try some of these tricks and give it about 3-5 days. If the bulb is still acting up, contact the vendor you got the bulb from, because it is then most likely a bad bulb.