Does the beer taste fine until a certain time? I know you said its been in the bottle for longer than a few months, but, are you drinking some of that batch and then after a while the rest of it starts to taste bad? Has the beer been sitting for a long time in the bottles before you even try one? Where are you storing it? Is it subjected to large temperature swings or exposed to light?
I'm just firing off questions from the top of my head to try and narrow it down.