My, new to me, 2000 Powerstroke concerns me at startup. Sometimes, when I turn the key, it starts right up, turns over at a good speed and cranks right up. Other times, it seems to spin slowly and actually sounds as if the batteries are giving up. If I stop, switch it off, reheat it and then try to start it, it fires right up. The starter spins faster and everything. It doesn't do this every time, usually first time in morning or after it has sat for a while. The batteries are new, 650 amps.