Fine moving detail can expose compression limits even when the stream arrives continuously; this is not the same as buffering.
Why this happens
A football pitch, a large crowd and falling confetti contain many details that change from frame to frame. A compressed source has to represent that information within its encoding choices and available bitrate. During difficult scenes the picture may lose texture or show blocks while playback continues at the normal pace. A sharp static studio shot can therefore look better than a fast sports pan from the same nominal resolution. The dimensions alone do not describe how much original detail survives compression.
Look at whether the blocks track busy movement and disappear in simpler scenes. If so, compare another quality version of the same programme rather than increasing television sharpness until edges become harsh. A faster internet plan cannot add information omitted from the transmitted video. Network interruptions can still occur at the same time, but keep them as a separate observation: a freeze, a quality change and persistent compression artifacts are different symptoms. A short description of the scene and source version gives much more useful context than an unsupported claim that every HD channel should look identical.
A useful way to work through it
Compare a busy moving scene with a simpler scene on the same feed.
Keep visible compression artifacts separate from spinners and frozen playback.
If available, compare another source version without changing the TV processing.