Mine did this when the O2 sensors went.
Ran OK under light throttle, idled like a dog and overfueled under load.
It would cough and splutter when I tried to put the boot in.
Also check the pipes around the top of the engine, the old ones used to perish at the bends at the back of the engine, escaping vacuum and all that...
More of a read here...
Australian LS1 and Holden Forums