92 diesel starts right away, then dies and will not start

My truck, with 146,000 miles runs great after it starts, but will not on occasion. It always starts immediately, then sometimes dies. After a while it always starts, but sometimes it takes many minutes of turning over to do so. Then, it shudders as though it is not getting enough fuel. It seems to be a lack of fuel, as the gases coming out of the tail pipe are white as it turns over without starting but the mechanic can find nothing wrong as all the tests turn out fine. We replaced the fuel pump cellonoid but it did not help. I should add that it has never NOT started for the mechanic.