Preglow mystery

An unusual problem (to me) with my 1976 300D has developed over the past few months. Engine 617.910. Body 115.114. I have owned this car since 1984 and it has 133,000 miles.When I turn the ignition key to #2 position the pre-glow indicator lamp comes on and stays on too long. The glow plugs do not heat and when I check plugs (input and Output) with volt meter the meter indicates they are all getting current. Then I replace each plug indicidually until the engine starts. The normal assumption is a bad plug. The next day I can put the plug back in the engine and the engine starts. I am bewildered.