If after replacing the tube and starter, you still have problems (unless one of these replacements is faulty), then the most likely culprit is the ballast unit (inductor/choke) contained within the light fitting. Replacements are not readily available.
My advice would be to test both the tube and starter within a known good light fitting – to eliminate the possibility that you have purchased faulty items. Then replace the light fitting – electronic ballast units are now available that do not have a starter (that needs regular replacement) – and no doubt the manufacturers claim better efficiency over conventional ballasts.