My 1999 new body style, 4.8 vortech 2WD Sierra has an issue which has plagued it since I bought it a couple years ago.
When it starts fresh in the day, it runs fine, then after warm up, I make multiple stops all day long, and often after warm up, when I have been stopped only a few (5-!0 minutes) the first 30 seconds, it wont fuel up, meaning I can only do very light throttle, if I step on it hard it chuggs and backfires, then it clears up in less than a minute and runs fine.
Vapour lock?

 

Try cleaning mass air flow sensor first. Are you getting any codes?

 

Bad O2 sensors can cause these symptoms. The sensors are not used during warm up or open loop, but when it goes into closed loop they are. Have a shop with a really good scan tool read your computer. Mine did this and didn't throw a code. But the scan tool showed both sensors low voltage meaning they were bad.