Having the same problem on a 2005 4.3l....... replaced plugs, dist. cap, coils, and cam sensor. o2 sensor is good. idles great till you push the gas, then it misfires. timing was checked it was fine. could egr valve be stuck open? intake manifold gasket gone? ive spent almost 800 trying to fix this and im on my 2nd shop. want my truck back. she's a nice looking paperweight, but not what im looking for......

These 4.3 throttle body injected vehicle has few thing that could caues that concern the spider injector will burn inside the throttle body also make sure fuel pressure is within spec, fuel pressure regulator and or seal inside injector pack will fail and leak off pressue ,last but least make sure no vacum leak at intake manifold or throttle body

Something is probably leaning out the mixture when you open the throttle. It seems that any ignition issues have been ruled out.

So, check the following: fuel filter clogged, low fuel pump pressure, the fuel pressure regulator could be stuck at the idle setting, the EGR valve could be stuck open, faulty throttle position sensor (TPS) or circuit, faulty manifold absolute pressure (MAP) sensor, faulty or dirty manifold air flow (MAF) sensor, faulty intake air temperature (IAT) sensor, and an air leak into the intake duct or manifold between the MAF and the throttle body. I think if the intake manifold gasket had a leak, it would be misfiring at idle.

Ask the shop if any of the above checks have already been accomplished. I'll bet most of them have, but one may have been overlooked.