For sure your hardware is up to snuff, if we are talking local there is either an issue with your software setup/network/or client you are serving media to. Have you tried playing these same files through the web browser on the server itself. Or through some other device.
You may just be throwing money away buying new hardware.
Yes they play fine on the web browser, just not anywhere else. Tried local on mobile, fire stick, roll. Roe more on iPad, and mobile. Always the same. Used to only have stuttering when playing large 4K files, after upgrading to premium I had the stuttering constantly regardless of file type. I’ve turned off GPU acceleration for now