Engine wont start up?

Hi ! Most days my car starts first time, other it takes 3 or 4 tries, this doesn't matter if its warm, cold, hasn't been used a day or two, or has been used just an hour or so prior. Its very odd. When it does start its sounds as though the first 3 or 4 seconds are fine, then it dips off as if its going to cut out before picking itself back up again then driving fine. Cannot get my head around it. Has had a new spark plug hose yesterday with no difference. Any ideas? Thanks, Paige xx