My Inspiron 7559 is an i7 model, with an added 8GB RAM stick.
Since the past week, games which used to run at 60 fps are now running at below 30 fps (Witcher 3 used to give me 50fps and now gives me 15-20fps).

I have already tried the following:

1) Disabling Game DVR in the Xbox app

2) Doing a clean install of  all drivers available on Nvidia's website

3) Installing all the above drivers after deleting the old one using Display Driver Uninstaller

4) Installing the driver available on Dell's website

5) Checking all the settings in the Nvidia Control Panel

As it sits now, the laptop is running on driver version 368.71 from Dell's support page.

Also, Here is what I've noticed:

If I launch a game immediately after installing the graphics driver, it performs at its actual 60fps level. However, 15-20 seconds into any game, the game crashes with no "XXXXXX Stopped working" dialog box. If I launch the game again, it then gives me the horribly low fps. This isn't specific to any game. The Crew, Rocket league, Witcher 3, Battlefield 4, Project CARS, you name it.

I am now at my wit's end on how to solve this problem.

Please help.

i7 6700HQ, 16GB RAM, GTX 960M