I bought a 1999 Chevy truck for parts,the truck is identical to mine,(but mine is a GMC),down to the 454 engine. I have changed the whole drive train. When the truck is first started it runs perfect!! After the cold start mode it blubbers,and wouldn't pull a VW out of a mud puddle!
If I feather the gas peddle it will smooth out,but if I press it to much it blubbers and runs erratic! The motor only had 38,000 miles on it,but did sit for a few years. Anyone have any ideas on how to fix this!!

 

Could be several electronic and/or smog items. But when my 454 was loosing power, I struggled a long time with the truck. Almost sold it, until a great mechanic found out that the bushings on the distributor shaft were worn. Causing the plugs to fire as much as 10 degrees off, very erratic. mine's a 1990 though.

good luck.