poor car perfomance

i bought my ford fiesta sport 1.6 in 2012 januanury and since then its been giving me problems, it swithces off automatically while im driving and i raised theses issu so many times when i took my car for service but still they told me they cant find anything. im so disapointed because this matter is getting worse. i would really appreciate if you intervene because i cant take this anymore, its too dangerous to drive this car