Try booting from a flash drive. You could have almost anything on a bootable flash drive: Windows repair disk; Windows installer; memtest86+ to test RAM; a Linux Live USB; etc. The point is to see if the system can be stable from a different OS, which could suggest that you have an issue with your Windows installation. memtest86+ could be good, because it will test your RAM and just keep running for hours/days until you stop it, and it will show you the system temperature. That could give you a clue as to whether your shutdowns are due to high temperatures. Your laptop model is from 7 years ago, so it's possible that it is overheating. At that age, the thermal compound has dried out and cannot adequately transfer heat away from the processor, and there may be dust inside the laptop and maybe inside the fan and heat fins. So, your laptop could be unable to adequately cool itself. That would cause the processor to throttle (slow down).
I have dell latitude E5570 sudden turns of windows when desktop shows up despite of i update bios and drivers and reinstall windows
E5570 Sudden shuts down