Starter/battery/alternator

When starting my 1995 Ford F150 XLT pick-up, the engine turns over, sounds normal, but won't fire. There is no metal clicking sound like the battery is low nor do the lights appear dim. This has happened twice in about 6 months. After cranking it on and off for over 5 minutes, it finally fires. Again this has happened on just two separate occasions within 6 months. However, I noticed both times this has happened it was at least 40 degrees and above outside. In cold weather, 20 degrees and below, the truck fires up the first time. And so after the 5 min of cranking, the truck was sluggish and I had to depress the gas pedal continually in order for it not to stall. Then once it got going I drove off around the block, came back home, parked, shut the engine off, then immediately tried starting the truck again, and it fired right up within 2 seconds. Could it be the choke, fuel pump, or airlock in the line?