I had some similar problems. I teared down the controller and cleaned all part really good with electrical spray ment for cleaning stuff like that. Waited for it to dry properly. It worked just like it was brand new. Try it. The spray is not expencive.
I spilled coke on my controller, but i ended up having the same problem as you. All buttons worked fine, but can`t run in COD. Now i`ve tried to disambly the hole controller and cleaned it good with electrical spray and dry cloth. Gonna let it dry for a wile and then try.