If you have one vacuum leak, you may have more less easily detected. If all you did is replace one hose may have to replace more. Best rule of thumb, if one needs replacing, replace them all. Have you checked what your idle vacuum is?
Also, it's a 2003, don't know how many miles you have on it, but you're pushing ten years, may be time to clean the entire throttle system. Ford Mustang IAC IAB - Solving your idle problems
I know it's for a 5.0 for a mustang, but the components should be similar.