To be honest, it could be anything, an AV backend deciding to do a randomly scheded scan, Windows deciding to reindex some soft-linked media folder, other software deciding to 'phone home' to see if there's one of the occasional and infrequent updates, your spreadsheet that you have open in the background is being periodically autosaved... Something that gets in the way of the (normally just about managing) pipeline from media file through to media-player, or that already 'knows' it needs (e.g.) about five seconds of lookanead of media stream to deal with buffering issues, but gets hit by maybe 5½ seconds of interuption dje to one lf many (including the above) reasons.
That's if it's not consistently stuttering at the same point in the same media playback, every time, when it might be codec issues (a permanently 'awkward' unpacking/rendering situation). But you probably would have noted that already.
What's your OS? (Esp. if not Win10, which I think we're presuming.)
What's your media player? (Windows's/whatever's default {whatever that is at the moment..}, VLC {might be worth a try, if not ready, to see if it's better/worse}, FF {ffplay, part of winff package if necessary, could be a tad more technical to use but plays pretty much anything that any other non-proprietry player can - processing power allowing}, etc, etc.)
Do you know if/how much hardware acceleration is being used, or is it all going through the CPU?
For the games, do you mean normal interactive action, as well as cit-scenes? (More inclined to blame GPU momentary underpoweredness, but also other data bottlenecking/timeouting could still be at play.)
Have you checked for CPU/Memory/Disk/Pagefile(=Memory+Disk) use occasionally spiking in any way? (And also looked for odd spikes when you aren'tviewing/playing, so that normally you'd not see the knock-on-effect.)
Are the media files *huge* BluRay rips at a 4k resolution that is overkill for your (high-end, but not 'home cinema'-quality) setup, or (conversely) are they low-res files which you're somehow on-the-fly upscaling to match your super-hi-res monitor/screen?
You're not screen/session-mirroring (VNC, TeamViewer, etc) or networking some or all the rest of the data-moving tasks (involving rssh, scp, etc..). (Probably not, but while I'm asking questions I thought I'd ask that too.)
...now see, that's why I didn't reply earlier.
You do really want a Friendly Neighbourhood Spiderman Techie getting their eyes on your problem[1], whether that's your warranty-agreed original supplier who may have to RTB it to do a 'proper' job, or the person with the reputation for being the nerdiest geek/geekiest nerd on your street who might know what's wrong just by popping by and listening to the harmonics generated between the CPU and PSU fans...
[1] Odds on are that it won't (mis)perform in front of suchban audience, though. That's a fundamental law of the universe..