I just got a 5K 2017 iMac. Streaming video from all the major services — Hulu, Amazon Prime, Netflix (non-original series) — looks terrible. Grainy, with a shimmering noisiness. Netflix Originals look pretty good, on the other hand, as do 4K videos from YouTube, but most series available on major streaming services look awful, worse than on the 2010 iMac I just replaced. Is there anything I can do to improve this?