My 1997 Ford F250 with a 7.3 Powerstroke is hard to start on colder mornings. If I cycle the glow plugs about 10 or 12 times it will start easily. If I plug in the block heater it takes fewer cycles to start. I've checked the glow plugs with a test light, they all have continuity. I've checked the voltage at the connection wire and have about 12 volts DC to each. What else should I check?

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