Q: Macbook Pro 15-in 2008 will randomly shut down when put to sleep

OS X El Capitan

Version 10.11.6

MacBook Pro (15-inch Early 2008)

Memory 4GB 667 MHz DDR2 SDRAM

Graphics NVIDIA GeForce 8600M GT 256MB

After updating to El Capitan my MacBook Pro will sometimes try and shut down when it goes to sleep. Whenever I leave my computer for around 10min the display goes to sleep. When I log back in some times I will get an error message saying one of the Applications I was using prevented the computer from shutting down. Some applications will have successfully closed down (forcefully). This may also happen when I close my computer. I don't know if it helps but my Energy Saver settings are set to:

Computer Sleep: Never

Display sleep: 10min

"Put hard disks to sleep when possible" - unchecked

"Wake for network access" - checked

"Start up automatically after a power failure" - unchecked

How can I prevent from this happening? I don't even know where to begin.

Thank you