If you experience a musty smell when you open an occasional bottle of your wine, yet it doesn’t happen with every bottle from the same batch, the problem is not with your batch or with your bottles. The cause of the smell is cork taint.
This arises when naturally dark corks are bleached to produce the lighter colour that most people prefer. When the cleaning agents used in this process mix with mould that might already exist in an individual cork, they produce a chemical called trichloroanisole. And this chemical can actually spoil your wine.
If you were to switch to synthetic corks such as Nomacorcs, you would never experience this problem again. They cost slightly more than natural corks, but they keep wine very well and eliminate the problems of taint and wine spoilage.