I too had this problem on my car. It was the fuel non return valve that is connected to the fuel pump and then the fuel pipe connects to it. It helps the accumulator maintain fuel pressure and stops fuel returning back in to the tank. I too went through the expense of trying many new parts like injectors, accumulator etc. The irony is that this problem was introduced when the previous owner replaced the fuel pump for me before I took ownership and the non return valve supplied with it had no spring and ballbearing fitted.