Hard start when warm 1999 Ford F-150?

I have a 1999 Ford F-150 that has a hard restart problem. When the truck is cold, it starts up no problem. When the truck is warmed up and I attempt to start it, it cranks about 15 times before it starts. I have already checked the fuel pump (running @ 34 psi) and the fuel injectors as I thought this was more likely a fuel related issue, but now I don't think it is because they are all working within spec. Does anyone have any ideas on what this could be?

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