vauxhall zafira cuta out?

I have a Vauxhall Zafira and have just had a new starter motor fitted. When it returned all was well but now after about a week or so it has starter cutting out. I tried some engine flush for the petrol and this helped it slightly for about a day. I checked the air filter and am wonder weathercheck on the plugs now. Any sojest

LeeAnn Hill
September 2011