76 240d glow plugs

[240Dude]

this morning i get ready to go to work and i can't get the glow plugs to going. the indicator doesn't glow, the car won't start and i don't have time to hang uot and check things, so i go to work in my other car and when i come home guess what. everything works fine and the car fires right up. at this point i don't really want to take anything apart until it fails again. am i wrong in thinking that if a glow plug goes bad it will stay bad. maybe the ground wire is a bit dirty. i tested voltage on the glow plugs by hooking up my dvm to each plug while pulling on the switch. the number 4 was only 10 volts and each plug after that dropped a couple of volts. i live in the sf bay area so it is not cold at all right now. any thoughts would be helpful.
john