punto cutting out?

hi all i have a 2003 fiat punto active sport 1.2 8v...car starts great every morning. when driving and i go to stop at say a junction or lights then the car will cut out. if i wait 2 or 3 mins then the car will start and drive. this can happen 2 to 3 times a day. has anyone got any ideas. i know that the previous owners had a new engine fitted as the old one was goosed. could the mechanics have knocked something while installing the new engine. its getting at me. i cant afford a garage as im unenployed at the moment. ive got a handheld diagnostics and no codes are showing. the car drives great apart from this cutting off problem..thanks.