Glow plug cycle issues

Hello I got an 87 f250 6.9 idi diesel. Glow plugs have been replaced recently by previous owner. I am having some issues starting truck in the mornings occasionally, but not all the time. A few weeks ago I went to start it and glow plug light only stayed on for a second and did not cycle. It also did not click. It had a hard time starting. Other mornings it starts fine and glow plug light stays on properly even when cold. Well this morning truck did it again but this time glow plugs stayed on a bit longer first crank. Engine started and died quickly. Turned key halfway on and glowplugs only heated up for a second again but this time it was clicking. Still had a hard time starting it today. This has only happened twice and am not sure what it is. Is it glow plug relay or glow plug controller malfunctioning? I dont want to buy both parts if I dont need to. Any help would be greatly appreciated and thanks again. My NEW email address is iannordman92@gmail. Com.