Well here's the thing. The reason I asked 4 and 5 was because, if your phone is NOT doing these random reboots on stock, there was a problem flashing the phone. Also, if it DOES do these random reboots on stock but with the apps installed, how would you know whether or not an app is causing these random reboots?
It's called trial and error... you have to revert back to full stock, no extra apps installed and test it from there. If it doesn't reboot, then you know it either had to be the custom rom or roms that you had tried out, or it was one of the different non stock apps that was causing your phone grief.
I'm not saying you shouldn't have any of these, my point is, uninstall everything and revert to stock, then test the phone out. If it doesn't reboot, you know one of those 2 things was the problem. Then simply flash a new ROM, test it out some more and see if it acts up again, and if it doesn't then you know it had to be an app that you had.
After that, it's simply the process of installing and testing out each app you have had previously, and seeing if the phone will reboot after the installation and use of any of those apps.
To find out the cause of a problem, you have to back track, it's the only realistic way to fix something. You can't just always try to return a defective product like this for the problem it's giving you, because in your case, you flashed a custom rom and hence voided the warranty and even if you DID return it, you would STILL have to revert it back to stock anyways.