Did you contact the manufacturer? If these fixtures are a constant problem, there's always the possibility that they have been installed in a manner they were not designed for. A simple oversight, such as proximity to the ceiling, too much load on a single circuit, (the possibiliteis are endless), etc., could be causing the fixtures to eat components. They could also contain a design flaw that the manufacturer has already come up with a quick fix for, and they can give you the run-down on that.
The other problems I run into alot are voltage fluctuations or other problems with the supply power (i.e. high harmonics), and heat.
Voltage fluctuations tax a ballasts more than you might imagine. Too high or too low voltage both have an effect on the coils and the caps.
Heat is always a problem too, but mostly for the ballasts. Once the enameled coil windings get too hot and start breaking down the enamel insulation, the ballast is done-for. Is there any ventalation around the ballast? Is the ballast too close to the bulb and not shielded? These are all things to look for...