As Larry indicated, I think the relay is likely the cause. The contact points burn out. You can open the relay box and try to clean the relay contacts and check all the soldering joints. If the 80A fuse is old, you may, as well, replace it too at the same time. The glow plug light is also a diagnostics tool by the way the light is turned on. The service menual has a section to check the system by watching the light. Also, the coolant temp sensor which is a part of the glow plug system may be faulty too.