Looks like macroblocking to me. I'd try NeatVideo, which is more a denoiser but I've had some luck in the past with it helping to hide compression issues from older Nikon DSLRs. www.neatvideo.com.
There are lots of other noise reduction plugins, you may want to experiment with several of them to see which gives you the best balance of perceived quality and render performance. You may also find that some work well with a soft-edge mask, to avoid having to render the entire frame. Either way, you likely won't get real-time playback without rendering.
Two last thoughts: make sure you're watching playback in 'Better Quality' mode, not 'Better Performance'. The latter may introduce its own issues. Also, do a test export using your intended delivery settings, too. I've very occasionally seen nasty interactions between footage issues and export compression options.
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