First, do try another cable between the TV and computer's graphic card.
Does not take much for a cable to cause problems and while it may work in one situation it may not work (or fully work) some where else. Check the cable specifications as well. May physically "fit" but the quality and construction may not match your connectivity/device requirements.
And if the cable came with the TV then it is likely to be as low end as possible.
See below per the following link:
http://www.digitalcitizen.life/what-screen-resolution-or-aspect-ratio-what-do-720p-1080i-1080p-mean
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Depending on the aspect ratio of your display, you are able to use only resolutions that are specific to its width and height. Some of the most common resolutions that can be used for each aspect ratio are the following:
4:3 aspect ratio resolutions: 640×480, 800×600, 960×720, 1024×768, 1280×960, 1400×1050, 1440×1080 , 1600×1200, 1856×1392, 1920×1440, and 2048×1536.
16:10 aspect ratio resolutions: - 1280×800, 1440×900, 1680×1050, 1920×1200 and 2560×1600.
16:9 aspect ratio resolutions: 1024×576, 1152×648, 1280×720, 1366×768, 1600×900, 1920×1080, 2560×1440 and 3840×2160.
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Have you tried updating the graphics card's drivers? Download from the manufacturer if and as necessary.
Try various TV menu display settings and likewise various Windows display (actually the graphics card) options.
Test very methodically by setting the TV at some configuration for example and working through the graphics card settings. Resolutions, etc..
Then change the TV to another option and then again work through the graphics card setttings.
If the pixelation continues then either the TV is faulty (with respect to being used as a monitor) or there is some mismatch that cannot be overcome via the available configuration options.
A bit tedious that is for sure. However, that is what will be needed to address problem and figure out what is going astray.