Did you replace the starter with a new or rebuilt one, or did you install a used one? Probably the most common problem with starters is worn contacts (actually just one of them wears). I mildly recall there being something else that can cause the starter to just click, but I forget what it is. Also, if the charging system isn't working properly or the battery is bad, the starter may not be getting enough juice. Usually that is not an intermittent problem and there are other signs and symptoms such as the charge and brake lights being on.