First fix your dangle (which I think you have from your previous posts) and verify that all your power and grounds are secure like everyone else has suggested.
I have had the same experience with my CarPC and it had nothing to do with wiring. Basically my problem was software. I had a bad device driver for my USB GPS reciever and my CarPC would BSOD (Blue Screen Of Death). Now, The screen will NOT turn BLUE if you have automatic restart with a mini dump enabled. The system will appear to turn off and then back on.
Your signature doesn't say what OS you are running. This response may be totally off your root cause, but hey, a similar sympton (Hard turn off and then boot up a few seconds later) happened to me and I simply re-installed my drivers and Wah-LA, no more crashing (reboot).
You can turn off the automatic restart which will then display a blue screen. Often times the BSOD will point to a specific .dll or .sys file that crashed. This is how I knew to re-install/update my dirver to fix the issue.
Anyhow thought I would throw in a different opinion/perspective on your issue.
Good Luck with it.