How It Was Fixed (Possible Software Bug Not Hardware): Getting the iMac Screen/Menus Garbled and/or Yellow Circle Before Getting Into Finder Issues?

Question

Hi. Possible fix done to my late- 2009 iMac's screen/graphics issue that may work God willing, hopefully for most [In my case it sometimes had that yellow circle that appears before it goes to the Finder but mostly it was a messed up desktop and menus (unreadable mostly) after months, prayers and weeks of fixing it; recommended to be read entirely first coz' it's a bit complicated (like a lot of subplots). If uncomfortable to try coz' it involved a GPU stress test, contact your authorized Apple service center for their advice]. I had an iMac (late 2009) screen garbling a few hours ago (was weeks ago when it resurfaced until it got worse that it's at every boot on Mavericks):
†        I booted to Snow Leopard USB external (my back-up OS) drive then run the first    person shooter at redeclipse.net. I set everything to max, including the display, resolution and graphics setting with fullscreen and v-sync on. I then refreshed it which brought it back to normal (I then tested it's stability with GPUTest 0.7 with Furmark for 15 minutes). * if the display options have been set to max before, bring it back to the default then refresh and then max it again Possibly any games that's has the graphic settings (or more advanced) of Red Eclipse could work (other free ones are Xonotic and Open Arena which are based on Quake 1 or 3s engine I believe).
Optional but not recommended (have warrantied if that's still available or have it repaired at authorized service centers):
If that doesn't work, review articles and videos on how to open an Intel iMac and get an iPhone/iPad/another way to get to the 'net ready for research (I opened mine very carefully on the right side with a cutter with blade extended far prying it carefully then I inserted a flat end, sturdy plastic used for mixing epoxy then removed the cutter carefully to let the plastic mixing tool do the prying) to see if the cable connected to the logicboard which is on top of, is connected (mine was detached but I'm not sure if the adjusting and tilting of the screen overtime for 2 & 1/2 years, detached it) then do † above again.
It could to be a software problem, a possible Apple ATI driver bug that takes overtime to surface (2 1/2 years it resurfaced because this started in my case after upgrading to Lion when Apple coded things mostly from the ground up. Snow Leopard didn't have this problem). 
Thank you for your time. Have a great upcoming sunday mass/great upcoming weekend. I hope this helps.
God bless, Rev. 21:4, Matt. 16:18

Answer 1

Does an external display also act in a similar manner as the internal one?
Some earlier Intel-based iMacs had graphic processor card issues; not sure what
the symptoms of the majority of them were, but some had odd patterns, others
went black, and some would range from OK to totally unusable. Among others.
So there is a chance the graphic processor may be going out; or just the circuitry
that controls the backlight. An inverter may be weak or other circuit to the display
could be wearing out.
http://www.apple.com/retail/geniusbar/
If you have access to an Apple Store with Genius bar, they can perform some kinds
of diagnostic tests there even though the computer is out-of-warranty; that may be
of help to narrow down the cause of the symptoms you've noticed. They may not be
able to repair it, however, at an Apple Store; as it is rather old by today's standard.
An Apple Authorized Service Provider could do repairs and testing on older models
the Apple Store's Genius and other may not be set up to handle due to vintage.
Find an Apple Authorized Service Provider
Visit an Apple Retail Store
There is a country locator page to help find either of the above in regions outside
of the US & Canada, not sure how the above links would work; and it appears my
bookmarks to the country locator page may not be accessible now to post here.
You may be able to get an idea about what kinds of parts support the display
function by name by looking into an iFixit.com repair guide for your iMac series.
Hopefully the parts supporting the display are the reason it is dimming down, as
a graphic processor failure likely would exhibit other behaviors than just dimming.
PS: I see you've added content to your thread after I'd started working on this
& the answer likely is a hardware repair; professional testing is worth the time
and you seem to have at least one other thread on the same topic....
Good luck & happy computing!
edited 2x