Please give advice!

1999 Chevy Silverado Z71 Ext.Cab. 115,500 miles. Nobody seems to freaking know what to do! When you start the truck after it has sit overnight it idles awefull. If you let it idle for about 5 minutes it picks up in the idle and runs fine all day. If I try to take off right after it starts, I get a split second hesitation like it is going to die and then it runs fine all day long. I have changed the spark plugs and wires, changed fuel filter, changed PCV valve, got all computer updates, ran B-12 and Seafoam through the gas tank, pulled of the little hose on the fuel pressure regulator and there was no raw fuel, and the fuel pump is fine. What could it be?

just because the pressure regulator does not have gas in the line does not mean it is good. Replace it.

Replace Replace Replace!!!!