to give you a clear view of how random this really could be:
i recently replaced my mobo and processor but then found my computer randomly froze and i had to reboot. i checked everything, literally everything, finally believing it was fixed when i took it home and changed the PSU.
it wasn't connected by the motherboard ethernet port anymore... as soon as i reconnected it (about 3-4 days later) it would again freeze every time i was doing something. put in a pci network card, it's fine.
you must test everything. very slowly. EVERY. THING. not just your main components unfortunately.
though it probably won't be an existing conflict unless something is just failing.
reboots are usually because of faulty connections between something. are you monitoring the temp at all times and you're sure it's now powering down becaues of that?
do these reboots happen only when doing something or even when idle/doing light work?