Macbook air 2011 El Capitan - Black Screen

This wasn't a problem due to spilled liquids for failed upgrades. It started with asking me to reboot repeatedly. I would cancel the pop-up, things would be fine for a few seconds then the pop-up would show up again. Periodically it would just go to black screen, but I could recover it by clicking furiously on the touchpad. That's no longer possible. I tried all the various resets, including booting into safe mode. The power button does nothing.

The only way to get any response is to disconnect teh magsafe power adapter and replace - then it automatically brings up the Apple logo and goes into a boot up, but without the chime. The progress bar moves along fine until about two thirds and then it slows down. Then it boots and everything looks fine for a few seconds but the reboot pop-up start coming up again. I've tried all the keyboard combinations. I've disconnected the battery and held down the power button for 10 seconds, then reconnected the battery – same thing. I have a bootable USB, but cannot get it to boot from this either. One time I was able to get the Internet recovery, but it stuck in the middle of the download. Any help would be appreciated.