So this just started happening today. I would turn to on, put my seatbelt on, clutch in then ignition and click, all power dies. Wtf.. So I waited for like 10 minutes then I tried again nothing. So then I tightened the already tight battery nuts and voila it starts. Now it did it again just now. Is it really just the battery cables or something else? The battery is only 3 months old, and it's no cheapo either.

 

There are several symptoms to be considered about the problem. I don't feel like typing too much information, but i found this, check it out...

https://www.rx8club.com/series-i-trouble-shooting-95/rx8-will-not-stay-dies-idle-214067/

 

If your battery terminals were loose or corroded then yes, the issue you described can occur.
Just make sure they are tight and clean.