Engine cranks but won't start.

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I just bought a second-hand GMC 2003 Savana. Drove it home, ran beautifully. Was very pleased. Got in it the next day to try to go to work, engine would not start. Cranks fine, lights, radio, A/C but engine only coughs a couple of times as I let the key go. I wait an hour, turn the key, starts up fine - as if nothing was wrong an hour before. Next day, it does the same thing - won't start. Crank sensor, fuel pump?