Replaced starter 'n now the car runs rough...now what??!!

What is your question?

My battery is good, the starter bench tests good, but the car will only start once in awhile. Other sites suggest testing 'the relay'. What relay, where is it and how do I test it? Okay, now I went ahead and replaced the starter even though it bench tested as being okay. Viola' the car starts, however it runs a little rough. Any ideas why?