Do you have any other symptoms? Word finding problems could be caused by mini strokes, by a neurological infection, or even a circulatory problem. This is a good one to talk to a doctor about.
This may sound crazy but are you taking any kind of prescribed medication that you have been taking for a long period of time? There are some medicines that if taken over a long period of time can do that... a friend of mine had that happen to her. She was clear headed and acting fine but never could remember anything .... just a thought