My Plastimo range no longer stays lit without holding the burner knobs in. Correct functioning is that one holds the knob in to light and continues to hold until the burner warms up; when the knob is then released, the gas supply stays on as long as the burner stays hot. Now, one burner knob never stays lit without holding it, the other takes several minutes. Does anyone know what is wrong with this? Where can I get necessary repair parts?Thanks in advance,jv