Try running the Apple Hardware Test by booting from the 2nd DVD which originally shipped with the laptop from the factory. See if any hardware issues are detected.
Run the third party app DriveDx (free trial period) in order to check the health of the hard drive. Post the complete text report here using the "Additional Text" icon which looks like a piece of paper. If DriveDx shows any "Warning" or "Failing" notices, then it means the hard drive is worn out or failing respectively.
If the drive health report is good, then use Disk Utility First Aid to verify & repair the file system. Start with scanning the physical drive, then the Container or volume. If there are errors, then run First Aid again until the errors are gone. If after several attempts the errors remain, then you will need to run First Aid while booted into Recovery Mode or from a bootable macOS USB installer. Once the errors are fixed, then try running "Repair Permissions" on the volume.
Also try booting into Safe Mode to see if that makes any difference after you confirm the hard drive is healthy. This will prevent third party software from automatically running at boot & login in case they may be causing a problem. Typical culprits are anti-virus apps, cleaning/optimizer apps, and third party security software.
I highly recommend creating a bootable macOS 10.11 El Capitan USB installer. If you have access to another Mac from 2007 to 2015, then you can use that other Mac to create the bootable macOS 10.11 El Capitan USB installer.
