Greetings amills, your situation is quite perplexing indeed. However, my initial thoughts were not oxidation, but rather a possible issue with your base malt and it?s affect from your water. Could it be that you have had a run of either bad, or sub standard, base malt that you only use, or use more of, in your darker beers?
Furthering this thought pattern, could your water have changed and now you?re realizing an issue? I?m sure you have a put great deal of thought into this problem, but sometimes issues such as this are better solved by stepping outside the box and looking at all the commonalities between styles as they relate to your complete brewing process and ingredients.
Just some thoughts.
Good luck, I hope you get on track soon!