faulty car

I bought a car from princes benoni about a month and a couple of days ago, since then the car has stuck two times on the same problem . The car just switches of and locks the electric windows and turn of the lights while I'm driving .when the car was a 2 weeks old and it had this problem , I explained to them that this car is huge risk because it might turn off whilst I'm on a free way and it would create a huge accident .but despite all of that they dodged by saying the car was ok when it was at the dealer. I did not expet this from a respected dealer like mastercars and I don't know if they are waiting for me to be in an accident before they take this seriously. I do not trust their car aymore , icant travel long distancces for which I bought It for,I am so inconvenianced and at the sametime I'm paying my instalments .I'm really dissapointed.