Where did you buy the fixtures? I've had a similar problem with fixtures I bought from a retailer online, but they were only $35 a pop. I have bought one from a LFS for double the price in the past and had the bubls last me for years with no problems at all. A friend of mine has also bought the fixtures from a LFS and has had no problems.
The online retailer shipped me several replacement fixtures and each one of them had their bulbs burn out within a few months. The bulbs also get the black ends pretty quickly. I figured I'd save money buying 2 $35 fixtures instead of one $70 fixture, but with all the bulb replacing I've had to do I haven't saved much. I'm giving up on flourescent fixtures because of these actually, I'm gonna try my hand at DIY led.