I think most of us who smoke cobs have had one go sour or burn out. Part of the learning curve. Fluffy pipe cleaners really help to stave that off. Use at least one to keep things dry each time you smoke, and I like to leave a fresh dry one in the stem overnight to help wick up moisture/tar.
My first cob began to taste off (like a stale cigar) a few months ago. If I didn't have a sentimental attachment to it, I would have just thrown it away. But I sanded the cake almost completely back to bare "wood" and used a lot of Everclear-soaked fluffy pipe cleaners, bent and twisted to increase the contact area, in the shank. It didn't seem to work at first, like it was just bringing more and more dormant foulness to the surface, but we're in a good place again now.
P.S. Be careful using alcohol in the stem. I've had it soften and crack from leaving it to soak over night, so maybe don't do that.