any suggestions?

i own a late 1990 (or early 1991) pontiac gran prix with a 3.1 fuel inj. v.6. it runs and drives fine, but at low rpms it idles really rough, and sometimes it dies... i have a bad catylatic converter... can that be what is choking it out? only at low rpms though, when throttle is applied, it is fine... honestly, i just want to know what made it do that... someone help