All of a sudden my MacBook Air has started continuously rebooting. The screen cuts out, it goes back to apple logo then log in prompt. I can log in and then after a few seconds typically it cuts out and process repeats. Sometimes it cuts out before I can even log in.
The time is quite variable before it does cut out and restart.
I booted to safe mode and have run disk diagnostics about three times now, every time it comes back all fine.
I panicked and finally bought a dedicated external hard drive and did a time machine back up which completed successfully.
After this I rebooted normally and things seemed fixed, I was using it for about 30 mins with no reboots until an app which switched to full screen caused it to cut out and the same behaviour to start again.
If it's not the disk- what could it be and where should I go from here?
MacBook Air 2013 - os ex El Capitan