I turn on my iMac, after normal use for a few minutes, sometimes when a new window or app is opened, little colored squares fill the display. Everything freezes. Sometimes colored lines slice across the display. The little squares flash on and off in different locations all over the screen. It’s driving me nuts and making my daily driver of a computer useless!
iMac 27″, macOS 10.12
What is the exact year model of your iMac?
Identify your iMac model - Apple Support
From you description is sounds like an overheating and/or failing GPU.
At this point, you have a couple of options.
1) Find an Apple Authorized Service Provider that is willing to service older iMac's.
Genius Bar Reservation and Apple Support Options - Apple
2) Replace what sounds like a very old and failing iMac with a decent used or new Mac.
https://eshop.macsales.com/shop/Apple_Systems/Used/Macs_and_Tablets
What is the exact year model of your iMac?
Identify your iMac model - Apple Support
From you description is sounds like an overheating and/or failing GPU.
At this point, you have a couple of options.
1) Find an Apple Authorized Service Provider that is willing to service older iMac's.
Genius Bar Reservation and Apple Support Options - Apple
2) Replace what sounds like a very old and failing iMac with a decent used or new Mac.
https://eshop.macsales.com/shop/Apple_Systems/Used/Macs_and_Tablets
Have you blown the dust out of it lately?
iMac freezing after dozens of little squares flood the display!
