Just a thought, are you using tap or distilled water to mix up your developer. Tap water in some areas could have minerals in it that shorten the life of the working solution. Try distilled water for a batch and see if it lasts longer. Also is there is a chemistry department at your university? They often have de-ionized water on tap in their buildings. If you ask nice I am sure you could get gallons of it for free. After all a darkroom is really a chemistry lab.