I have an 1986 300SDL which runs beautifully with few exceptions. One of which is that the glow plugs seem to work intermittently. When I turn the key to the "on" position, the 5 second buzz always occurs, but the dash glow plug light comes on only occasionally. If the light does come on, the car will start easily. If the light doesn't, it's a pain in the neck to start and may take several tries, while blowing big black clouds of smoke. Any ideas how to ensure that the glow plugs always work?