If the battery voltage drops under 11 volts or so during cranking all sorts of weird things happen. I'm pretty sure that's what my buddies car was doing, spark but no fuel, but I'm going off memory. It's a simple thing to check though. As a test next time it happens plug in a battery charger and see if it starts. Like I said before his seemed to turn over normally, not slow at all, but it wouldn't fire.
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