Bear with me here for a second...
I have an early 2008 MacPro3,1 that has been my work horse for many years and for the last several years has been running continuously as the home server. I've noticed in the last week that if I bang the desk that it is on, the machine will cold reboot immediately. (how did I discover it? hitting a stapler for big stack of papers)
Machine hasn't moved in quite some time and wasn't sensitive like this until perhaps the last week. Everything should be buttoned down well in the machine except perhaps the Bluetooth adapter on the internal mini PCI port, but the WiFi/Bluetooth is working just fine. Also, power cord is snug.
So I suppose the question is whether or not there is something in the PSU that might be sensitive to movement or perhaps something else in the machine? Other than this, everything else seems fine with the machine.
UPDATE 2017-03-14: One other thing to note is that when it reboots, I get the startup chime followed by ~10 seconds of no activity and then another startup chime and then it goes into the full boot routine. Almost feels like a CMOS reset or something like that, i.e. is there a reset button on the motherboard that perhaps is a bit twitchy? (no, I haven't re-seated things yet since I haven't had the time, but I did open the case to check that the two cards, the WiFi/BT module and memory risers are snug).