Help needed

Help needed

Some months ago I bought a second hand Canon lense 24-105 L f:4 IS. It looked great and worked great. But after some shots I saw that it has a serious problem. I searched the internet and after some time I found that other people also complain about it, but not that many to damage its stats on serious pages and shops. So maybe only few of them had this problem. Many people called it diffusion, others lack of sharpness. They claimed that it was reduced or even solved, by stopping down the lens. I saw small difference when I did use a close aperture and anyway, what’s the use buying a lense with IS and f:4 if you can’t use it at night? By the way it does this thing with all the digital cameras I use, but then again there are times that the sharpness is back !!! In either case I see through it crystal clear, but I get these lousy results.

Anyway, I am posting this photo today, that reveals the problem quite clearly. Any ideas of what it could be, and if it can be fixed by me, would be greatly appreciated.