Q: IOS El Capitan - Computer freezes

Are the crashes kernel panics? OS X: When your computer spontaneously restarts or displays "Your computer restarted because of a problem." - Apple Supp…

If so, open Console and show the log list if it is not already showing.

Find System Diagnostic Reports.

Click the little triangle next to it to show all the reports under that category.

Find the first one that starts with Kernel and select it.

Copy the text from the panic log and paste it into a reply here.

Foe future reference, IOS is the operating system used by mobile devices such as iPads and iPhones.

It could be a GPU problem. Do you have an Apple Store nearby where you can have it checked out? If so, make a Genius Bar appointment.

I don't know of any third-party software that would cause the video to freeze, so I don't think it would be something you have installed.

If it happens often enough, you could try booting into Safe Mode and see if the problem recurs. Try safe mode if your Mac doesn't finish starting up - Apple Support

You can try to rule out third-party software by booting into Safe Mode and testing. It's not particularly fun to work in Safe Mode, so unless it happens frequently enough to detect in several minutes of use (or you can have it sit running a youtube video and check back periodically), then it may not help the diagnosis.

Even then, some things are not functional in Safe Mode, so if the failure mode of the GPU is not active while in Safe Mode, you could get a false sense of the cause.