iMac keeps rebooting El Capitan 2015 won't start

Hey Guys and Gals,

I've got a mid 2011 27-inch iMac with 4gb of mem. Installed el cap the other day and all seemed good until today... I would come in to the shop in the morning and the computer would be having really strange problems with wifi, was taking 15-30 min. to connect. Today it was doing it. I walked away. I came back and it seemed done. I tried to go to a website and opened mail at the same time and they froze, the spinning wheel popped up... I walked away. Came back 20 minutes later and no progress. I held the off switch and shut it down and walked away.

When I turned on the computer the apple logo pops up as usual and the progress bar begins to load. It gets about 25% complete and it reboots, gets about 25% complete and it reboots, gets about 25% complete and it reboots... You get the idea. So.

I installed osX elcap on an external drive and I'm currently running the computer off of that.

Before I installed OS X on the hard drive I read some things and tried some things:

- holding Shift does nothing. Reboots at 25%.

- holding command S does nothing. Reboots at 25%.

- holding command SV does nothing. well not nothing but it scrolls some text but it will just reboot all of a sudden and problem persists.

- holding command R loads the disc utility window.

When I run First Aid I get this:

Take a look at the windows below.

You can see that I'm only using about half of the storage but it also says I'm using all of the capacity and that zero kb are available. Weird eh?

Is there any way to back up the info? I should have most of it backed up but certainly not all of it.