Are you using the 2.4Ghz or 5Ghz WiFi bands? 5Ghz can have advantages with streaming speed (depending on your immediate neighbours) but 2.4Ghz has better range.
The pixellated pictures are a different issue - the internet connection only carries your EPG (TV guide) and other online services, your TV is carried on the coax cable, and picture breakup is a classic symptom of problems on that cable or in your box somewhere. Are there any specific channels affected? (incuding HD/SD)
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