You didn't say how you checked the fuel lines. You may need to do more thourough testing, making sure to duplicate the conditions in which the problem usually happens. There is a dye you can add to the gas that will flourecse under UV light.That way you can find the leak visually even after the gas has evaporated. There are a lot of places to buy leak detection dyes. Here's just one of them. http://www.wescotools.com/c-121-leak-detection.aspx