Slow starting

Hi, I have a K3500 crew cab dually with a Vortec 7.4 liter (454) engine.
It has 105,000 miles.
When the truck sits over night, it starts immediately with no hesitation or starter whirling. Bump the starter and it starts. If, after driving or running it, it sits for up to twelve to fifteen minutes it starts immediately, no hesitation. If however, it sits for more than fifteen minutes but less than eight hours, the starter whirls the engine around but the engine does not start for about ten to fifteen seconds. I have had the fuel pump and filters replaced. They have replaced a vacuum hose that was damaged and replaced the spark plugs. They (professional mechanics) tell me the distributor cap and rotor cap are fine. They are clueless at this point. This problem has been ongoing for about 40,000 miles. I typically run unleaded regular fuel, but about every ten tanks run one tank of premium. My repair shop has a mechanic who has a 1998 3/4 ton Suburban with the same engine which has the same problem and they cannot fix it either. They have told me to just live with it. I would really like to get this fixed. The truck did not do this when I bought it and it had 12,300 miles on it when I bought it. The problem did not start until after about 60,000 miles.
No codes are being thrown nor are any indicator lights on I.E, check engine, etc.
Help please! Thank you!