Battery issue? A bad or failing battery could cause sudden shutdowns, but I think you've ruled that out.
Faulty Motherboard or Power Circuitry? Is the PC getting the right amount of power? Have you tried another PSU?
Thermal issues? Have you blown dust from the vents and fans? Take the bottom plate off and make sure its all clear.
Power Management & BIOS Issues? Have you updated the BIOS? Checked Power settings in both "Settings" and Control Panel?
Driver issues? Have you updated the Intel DPTF (Dynamic Platform and Thermal Framework) driver and also chipset drivers? Lenovo can screw these up royally.
Turn off "Fast Startup? Some Lenovo models don't play nice with Fast Startup.
Lenovo sucks unless from day 1, you blow the BS OS they put on it away and install a fresh copy of Windows 10/11.
The garbage they infect it with causes way more problems than it should ever have.
