[V8] smell/idle problem

[JOHN DODD]

This problem going on now for 2 months,here is the latest:
Review:the car starts OK ,runs OK for 1 min or 2 then seems to have a slight miss.
When in gear the rpm changes between normal and 300 rpm.Seems to have slight surging too when driving.
Latest is this exhaust smell(not rotten egg)more like a burned smell(oil or some foreign material).It was dark so I could not see the smoke colour.
My question is could it be the catalytic converter going bad ?
I did manage to change the vacuum hose on the FPR last night.Both the FPR and OXS are about 3 years old.I got a new OXS but most of you think it's not my problem.
Hope it's not something more seious with the engine.