I'm surprised you have to change a fluorescent tube on such a regular basis.
Most tubes have blackened ends by the time they need to be changed.
As long as the starter and choke are starting the tube ok I can't understand why this problem occurs.
The only thing I can think of, is that the tubes you are using have short life filament heaters.
Have you tried a different make of lamp?
I generally use Thorn tubes.