While LinuxCNC is running, open a terminal and type
sensors
Check the CPU temperature, if it is over 60 degree C, check the fan and remove the CPU cooler, clean and repaste the CPU, test again.
Also visually check the motherboard capacitors for bulging and/or leaks, if any are bulging replace the PC.
Same as above for power supply, if capacitors are bulging replace the PS.
In BIOS, disable hyperthreading (this is a must), disable turbo, speedstep, PCI-E aggressive power management, modem, sound, TPM, secure boot, etc, anything regarding power saving.
ASUS ROG Crosshair VIII Impact - AMD X570