2000 Other Chevrolet Models 2000 Chevrolet W4500 - Engine He

I am at a lost on this issue. From a stand still every once in a while (happening much more frequent as time goes on) when I get on the gas lightly or heavy I would get a hesitation sometimes for up to 3 seconds (or more) before the vehicle would take off like it should. Also when I put the vehicle in reverse I've noticed what seems like the idle speed possibly fluxuating (not sure how much but at night when the lights are on you would see the lights dim as you hear the motor "surge") and it appears to be getting worse over time for both issues. In drive it is not nearly as bad and happens here and there. The tranny was just rebuilt just last year, fuel pump replaced about 5 months ago, just replaced fuel filter, plugs, wires, coil, TPS, MAP sensor and exhaust man. On both sides due to both being cracked. I have check fuel pressure and the lowest I saw it when driving was 52 psi but average is 55 psi. I have cleaned the mass air flow sensor (everything looks good from a visual inspection after cleaning) and looked at the electronic egr valve and the valve appears to be moving free. Please help any info would be great. I don't get any codes and don't want to just throw parts at it. The vechile is a Chevy W4500 that has a 350 motor and a 4L80E tranny. It is basically a Isuzu box truck but with Chevy parts and logos etc. Any help would be awesome for I don't want to get in a accident because of a hesitation issue and not being able to get out of the way quickly enough. Oh and by the way the vechile drives just fine once I get past the initial hesitation from a stand still.