crwdns2934241:0crwdne2934241:0 stevenjklein

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The MacRumors website has detailed instructions for upgrading the graphic card on your model.  (I have the exact same model, with the exact same problem.)

There are actually several compatible graphics cards, but each has a few issues.

See here for the full details:

https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/?post=17425857#post-17425857

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