head scratcher!!!

Make sure the points where the negative cable and ground straps connect are clean and paint free and that the bolts are not all greasy. Also, what cables did you install? The factory cables are smaller diameter wire and it's not really common knowledge but the jobber parts places.. Like NAPA and Car Quest, carry better cables. You need to ask them for the largest diameter cable you can get for your car. Usually you can get one or two gages larger in the exact same cable and your starter may just need more juice. Do either of the cables get hot when it's turning over slowly?