My ongoing nightmare with Core Group S.A

I am so frustrated with core group South Africa - I purchased a new Mac book pro in January from the Istore and after only 3 months I had to send it back as it was shutting down and restarting on its own because of an unknown error. I took it down to I store pavilion with an explanation to the technical team on how how it was shutting down etc and they sent it off for a “repair” with a promise of a turn arpeund time of no more than 7 days...-exactly three weeks later I got a message to collect my Mac , but once I got my Mack back and later that day , I notice it still had the same problem! So the next day I returned it back to the same I store. For a second time now I explain how it was shutting down , thinking surely now core would be able to identify the problem , but no, a few days later I receive an email from Core saying there is no problem, so I immediately reply to them explaining how and under what circumstances the error will occur to hopefully help them see the problem, I also attach video and pictures with date stamps etc. but what adds to my frustration is that all along that I have been emailing them it has takes them at least two days to reply. Now after sending a detailed email and pictures etc I get an email saying that they can not download the pics I sent because of the format they were sent in , which ironically were sent form my iPhone . So now for the fourth time in as many weeks I will have to visit the istore again to see if the managemet there can escalate my query ! Surely the the technical Department have the required s****s to diagnose a problem efficiently. Apple products are supposed to work and I have always purchased apple products , but the level of service I have received from core has been appalling. Hopefully my issues will be resolved soon, because on the 1st of May it would have 1 month since I returned my Mac.