Slight engine surging.

[muncher]

Hi All. I will try to describe as best I can my issue. I have a V8 305 GMC Sierra. It has fuel injection even though it looks like a carb. When engine is cold it drives smooth. However when it is warmed up I notice that when I am maintaining a constant speed. The engine feels like it surges slightly. Almost like I am accelerating on and off lightly. (Hope that makes sense). If I accel or back off it is fine. Only happens when warm and only when I maintain a constant speed. To me it feels like a vacuum leak? All the US smog equip is connected. We don't need that here so if that's faulty I can disconnect it.
Hope you can help............Cheers.