The 21.5” iMac’s use the Intel iGPU logic within the CPU chip. Here’s the systems specs with the [https://everymac.com/systems/apple/imac/specs/imac-core-i7-3.4-27-inch-aluminum-mid-2011-thunderbolt-specs.html|Apple supplied 3.4GHz i7 (2600 S) chip] so you have installed the correct chip, as you put bad the i5 and the display problem went away it’s likely the i7 chip is bad! The only other possibility would be the display cable tugging on the logic board connector so the display cable data connection was not stable.
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The 21.5” iMac’s use the Intel iGPU logic within the CPU chip. Here’s the systems specs with the [link|https://everymac.com/systems/apple/imac/specs/imac-core-i7-3.4-27-inch-aluminum-mid-2011-thunderbolt-specs.html|Apple supplied 3.4GHz i7 (2600 S) chip] so you have installed the correct chip, as you put back the i5 and the display problem went away it’s likely the i7 chip is bad! The only other possibility would be the display cable tugging on the logic board connector so the display cable data connection was not stable.
The 21.5” iMac’s use the Intel iGPU logic within the CPU chip. Here’s the systems specs with the [https://everymac.com/systems/apple/imac/specs/imac-core-i7-3.4-27-inch-aluminum-mid-2011-thunderbolt-specs.html|Apple supplied 3.4GHz i7 (2600 S) chip] so you have installed the correct chip, as you put bad the i5 and the display problem went away it’s likely the i7 chip is bad! The only other possibility would be the display cable tugging on the logic board connector so the display cable data connection was not stable.