When you turn yr key on you should here a click of the glow plug relay and the light should stay on for about 15 secs or so, make sure the glow plug rail is clean and the glow plug nuts are tight. Make sure the power wire back by the firewall is tight. Like said by BTI said its prob the relay, unplug it and plug it back in and see if it starts better that way. If so just replace it....