It is 13 years old and has 19,000 miles on it. Your free hub maybe slipping. If you can try a different rear wheel, then you might be able to isolate the problem. I have never taken a free hub apart but if it has dried grease inside it may need to be cleaned and greased. If you can get some thin oil into the free hub it might dissolve the grease enough to free up the pawls and stop any potential slipping.