My son recently bought a 04 G35 coupe a couple of months ago. It is a manual coupe. A couple of times he has opened the door and had no power in the car. No interior lights, and the car does nothing when he tries to turn it over. No click, nothing. He gets out and waits 5 minutes and then gets back in and the lights work and the car starts and drives like normal. The battery shows 12.5 volts between attempts. Does anyone have any ideas as to what is wrong?

Welcome aboard!

Sounds like an issue with the battery - even though there's 12v showing at the posts, that doesn't take into account the cranking amps. Test a new battery and see if that resolves it. Also, a poor or intermittent ground can cause havoc with these (and most other) cars. Make sure ground cable is free of corrosion and has a good metal-to-metal connection at the frame.