Cursor artifacts and System freeze in iMac late 2009 - update how?

Hi, this is my 1st question here :)

Recently bought a used 27" late 2009 Quadcore i5 iMac with ATI Radeon HD 4850. Lush 12GB RAM and a WD replacement HD. Sierra was pre installed, so for 2 weeks I did not encounter any problems.

Midst watching a browser video, my bf encountered a grey screen of death. Upon restarting, the sys was reluctant to start. A never seen artifact showed under the cursor which looked like this:

Block Image

Forums are full of similar probs so here is what I tried:

> Sierra, Mavericks, Snow Leo clean install including HD wipe.

> All startup tricks like PRAM resetc etc.

> Hardware acceleration turnoff in Browsers

> ditch Safari altogether

> SMC Fan control

Once this artifact shows, the sys freezes, Programs unresponsive, hard restart, sometimes comes up, sometimes doesnt. Safe mode works 95%.

He seems to be fine for the moment running his native Snow Leo, SMC, Acceleration off, no Safari, no Flash or Java additional installs but also had 1 Grey screen so far.

Here is what I DIDNT try so far:

>Kext driver replacement for Radeon (no AMD support)

>Replace Graph card

>bake in oven or soldering

Forum advices seem to split up at this point, some say replaced Radeons show same issue after few months again, some say its even the whole motherbard, some say they baked some stuff in the oven, some say soldering points are the culprit. Some say to take the 2011 Radeon 6 series, All of it is a number too weird for me.

What t feels like is moders systems are too daring for the old hardware, overclocking style.

So the basic question I have here: I would really like to use any Sys between Mavericks and Capitan, so how do I get the machine fit for it? Are any of you into regular Mac repair, have seen a similar think, know a good fix?

My early 2008 NVIDA in iMac 24" died, so I had a Radeon 2600 instead. On modern Sys still shakey but running well in 10.7. But this time I dont want to downgrade too much for graphic design.

My best running sys is a macbook core 2 duo mid 2009 on NVIDIA GeForce 9400M 256 MB and Mavericks. Is this at all comparable to an iMac? Is NVIDIA better for modern Systems?

Which GRAPH CARDS are usabe INSTEAD another 4850 (out of sales) or 6750 (just for gamers, overclocking and hot) ... any NVIDIA replacements?

Thanks so much guys!!!

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Only 1 same style artifact found: https://discussions.apple.com/thread/479...

ATH not run yet, DVD does not mout. Etrecheck all OK but 1 line: User launch Agents failed: com.apple.CSConfigDotMacCert.

12 hours since launch (with sleep mode inbetween), 6 crashes (had wrong version fan control for sys) and 1 Panic. All is fine since then.

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Correct me if Im wrong, this is the part I need? Snow leo Shows even worse artifacts now :/

Apple P/N: 661-5314 MXM III, ATI Radeon HD, 4850 512 MB

Just scared of some reports here saying the problem came right back :/

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I would do a hot air reflow of the GPU, do not bake it, you have no control.

If the hard drive has been changed you probably have a run away fan issue and need the OWC thermal sensor fix. This:

https://eshop.macsales.com/item/OWC/DIDI...

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deatroy will be eternally grateful.
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