A collector leak is not going to affect your idle quality. If that were the case you would expect a rough cold idle with your headers open, right? I'd say that you have a minor carburetor problem, or perhaps your plugs may be a bit weak - hot enough when everything is warmed up, but a slightly weak spark when cold. Have you looked at your plugs for electrode color? Changed plugs since you got it? What plugs are in the engine?