Brand-New Gaming PC Suddenly Stutters in Every Game and Sometimes YouTube

I recently bought a custom-built gaming PC for €2,700 with a Radeon 9070 XT Nitro+, Ryzen 7 9800X3D, 32GB of DDR5-6000 CL36 memory, and a 1TB P310 SSD. The first couple of days were completely fine, with normal temperatures and reasonable benchmark results. Then, without any obvious change besides installing Steam, Chrome, Discord, and several games, every game began stuttering heavily every few minutes.

The stutter is unusual: the game freezes, briefly resumes for a moment, freezes again, and then continues normally. It happens in Crimson Desert, Rocket League, The Blood of Dawnwalker, and Kingdom Come: Deliverance II. YouTube videos occasionally stutter too, although much less often, and sometimes the audio glitches at the same time with a brief distorted sound.

I'm running Windows 11. So far I've used DDU to reinstall the AMD drivers, reset Windows completely, removed recently installed programs, checked temperatures on every component, and run sfc /scannow. Nothing has fixed the issue. Does this sound like a hardware, BIOS, driver, or Windows problem, and what else should I test?