iMac 2012 - black-and-white stripes, graphics lags, artifacts...
dear apple users, bought used imac, late 2012, nvidia geforce gt 640m, i5-3335s ?, 8 gb ram, 1600 mhz, pc3-12800, 1tb hdd. fine 2 weeks until turned machine off night , when came office in morning , started it, happened: https://goo.gl/jatmed ( https://we.tl/smlbb1kkck ) first thing did called apple support , guy asked me perform different tests, including plugging imac out of socket and, after several minutes, plugging again. nothing worked , ended installing high sierra. this, however, did not work either. so, took machine , went certified technician launched imac and, 'dismay', fine! no artifacts, no lags, no black-and-white stripes on screen. told me perform tests took him 15 minutes. after took imac office, plugged in same socket - ok.... several minutes. then, thought - ok, maybe something's wrong socket , plugged machine different socket. in 'configuration' right several hours. device in use until 9 pm when put '...