what are these graphical/screen refresh problems indicative of on my mid-2011 21.5 inch imac?

we've had our mid-2011 21.5 inch imac since 2013 (purchased used). it's had its hard drive replaced at least once, i cannot remember the issue, i believe it just stopped booting to OS properly?

anyway, within the past month or two i've had some errors on this imac that indicate to me that one or more parts are starting to go and may need to be repaired or replaced. i'd like to do that before it is knocked completely out of service. i apologize if my descriptions aren't "proper", i'm not very knowledgable of computers. (we also call this computer "the mac" in my family, sorry if it sounds corny!)

the first problem i encountered was small. in applications, usually chrome, after a period of time, a pattern of colored blocks would appear near the top of the screen, around the taskbar area. if you moved your cursor, they often changed or moved on the screen, and would usually go away if you opened launchpad or another app.

the second and more pressing occurred today. as i was using the computer, chrome crashed and nothing was responsive except the cursor. then, the screen turned to black, with again, only the cursor appearing, and it was responsive. i pressed the power button to shut down and turned it back on again after ten seconds. it was very slow to boot and when it did, the log in page had a huge chunk of white bars scrolling on the screen. when i logged in, it appeared okay, but opening any application resulted in the page taking forever to load on the screen before my eyes, almost like how opening images on a dial up computer back in the day would show scrolling as more of the image loaded. this occured in chrome, when clicking launchpad, and when opening a group of applications in launchpad. in addition, typing any text resulted in a long wait as the letters flashed on the screen one by one, very delayed.

i tried to screen record the issue on quick time but it was not shown in the final recording, so i captured footage on my admittedly awful cell phone camera. apologies for the quality but it should get the problem across. the sound you hear is what i assume is the fan on the computer; i've never heard it before and it went away after i rebooted the computer.

i shut down the mac and powered it on again after ten seconds, and the issue seems to have gone away. given the age of the computer, however, i'm worried the problem is a result of some part which is failing or in need of repair. do you have any idea what exactly is going or what repairs should be done to solve these issues?

submitted by /u/nflez

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