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Bad/poor connection on the battery(s) or weak batteries? I'd load test both batteries...separate from each other which means you need to pull the cables on one of the batteries. If both check out good, then I'd clean the terminals on both batteries and check it again. Just don't continue to start it if the batteries are weak, you'll smoke the FICM if you do.
+1 on weak batteries. my old truck would do that as I had a bad batterie in it. even if you charge it and it starts the draining battery will make your alternator work over time and could burn up your FICM with inconsitant voltages, like ZHILTON said above.