1999 4.8L Sierra sluggish Backfiring

Hey to all, my problem is my 99 1500 Sierra idles fine runs ok if you don't press to hard on the pedal after which it will start backfiring, If you let off just a little it stops backfiring but has no power, RPM's go down to below 600, You basically have to pull off the road shut it off give it a moment and crank it up and go on, just dont press to hard on the gas petal. Cat's have been replaced,fuel pump replaced and filter, plugs replaced. There is no distributor on this engine. Does anyone have any ideas ????

Welcome to AF, check fuel pressure when the engine is loaded as you describe.

Hey again, I checked the fuel pressure this evening and while the engine was under load the gauge would read about 35 psi, while the engine was at idle the gauge would read 42 psi, with the engine off and key on the gauge would read 52 psi. My book says I should read with the engine off and key on, 55 to 62 psi. any ideas guys ???????