If you don't mind doing some detective work and minor surgery on the lens, you may be able to swap out the aperture detent springs for some stronger ones. There are specialty spring manufacturers in like where you may be able to track down some stronger replacements - although these tend to be expensive.
A less expensive route would be to add tiny spacers behind the spring and detent ball, but if the detent spring is weak already, this may not do too much to stiffen everything up.