I have had the same problem with the same model MacBook Pro except i had the 17". I delt with the problem for about 2 months when my battery finally died and when i took it to the apple store they replaced it under warranty. The weird part was that the inside of the battery had expanded and was bulging out. The people at the store told me that the batteries for this model when they first came out had a defect in them and when they expand they can damage your computer. Thankfully mine expanded outward instead of inward towards my computer saving my hard drive. Anyway i suggest you replace your battery before it gets too bad.