New PC freezes when idle but passes stress tests. What should I check?

Passing stress tests doesn't rule out instability at low load. As a diagnostic, load the BIOS defaults and disable XMP so the RAM runs at its default settings. Test for a while under the same light use that usually triggers the freeze.

If it still happens, try disabling CPU C-states temporarily and test again. Change one thing at a time so you can tell which setting, if any, makes a difference. You shouldn't need to pick an arbitrary RAM speed just to do the first test.

Check Reliability Monitor and Event Viewer around the time of a freeze. A display driver error, hardware error, or other event just before the reboot might give you a direction. A message saying the system shut down unexpectedly only tells you it rebooted, not why.

Since the screen goes black, also check that the NVIDIA driver is current. A clean driver reinstall is worth trying if the logs point toward the display driver.

You could check Windows system files too. Open Command Prompt as administrator and run SFC /scannow, then DISM /online /cleanup-image /restorehealth. If DISM repairs anything, run SFC again. It may not be the cause, but it's a straightforward software check.

Don't completely rule out the CPU or motherboard just because the stress tests pass. If the simpler checks turn up nothing, inspect the CPU socket and contacts for damage or discoloration, following the motherboard's instructions for removing the cooler and CPU.

Be careful with the socket pins. If you find visible damage, take clear photos and contact the retailer or manufacturer about warranty options rather than trying to force anything back into place.