Visual glitches when i launched Dota after giving my iMac for service. Please help

Here I'm elaborating to give more context. One year back i made the biggest mistake of giving my iMac to an unofficial mac service guy. Basically the only issue was that my iMacs fan was off and the system was heating. I had given it to the service guy and he suggested some more changes like extending the memory etc which I agreed to like a fool. I get the system back, and the Mac keeps restarting randomly, after few months it just wouldn't run for more than 2 minutes without restarting automatically. I gave it back and he says there is an issue with the mother board which he then later replaces and i get it back. Now when i launch Dota to test it out, there are lots of visual glitches that I have attached pictures of above. It seems bit blurry in certain screens. There is a black box around every hero and in random locations. Some areas also appear pixelated. When I pan my mouse down in game the map turns dark Can someone please tell me what the real issue is, I have tried visual settings and it fixes some aspects for a while but reverts to the same when i relaunch. I don't know if this is an issue with a graphic card or motherboard or what since I'm not a very technical person. I'm not even sure if the repair guy replaced the original parts with some cheaper versions of the same. Please someone help as what i should do next or what the problem could be? Thank you