99 4.3 starting problem

I Have a 1999 Chevy S10 4.3 Vortec. I am having a problem starting the truck when it sits for a while. It will crank over for several minutes. Once it starts and i run it, it will start all day with no issues. It only happens when it sits over night or for a long time.I can hear the fuel pump turn on, and it has been replaced. The plugs, wires, and cap all have been done. Tried a new battery. Had hours of diagnostics done and have gotten nowhere. I just dont get why it is only when it sits. Its driving me crazy.
 
Any Ideas?

Howdy, jrod332.Was the fule filter also changed?  try some fule injector cleaner/octaine boost , and a free code scan from the parts store. it might read out somthing intermittint, also check your ground wires to the chassy.

Howdy, Update on cold start or sitting a while, The Haynes manual pg 6-8 (IAT) Intake Air Temperature sensor, Inside the air duct in the filter housing acts like a cold air thermostat. I suggest the manual from a parts store for the rest of the details.
                                                                     

Definately could be a air thermo sensor problem as stated by sickvortec.