That is a good idea. Test it by taking the PC inside and running it off standard ATX supply for a bit.
As to where to start troubleshooting a pc that randomly reboots (note that you must run the pc for several days or until you notice it randomly rebooting again after each step):
1) replace (or swap w/ another pc) the ram. If that doesn't work, goto 2.
2) format and reinstall. If not fix, goto 3.
3) Power supply replace/swap. In not fix, goto 4.
4) disconnect everything from motherboard except hard drive and video (also remove all pci/isa/other cards. If not fixed, goto 5. Otherwise, one of those components is the problem.
5) cpu swap/replace. If not fixed, goto 6.
6) Motherboard may be bad.
If you can't underclock, you can try running drives off of another 12v power supply to relieve the opus a little, just trigger the source from the opus & you'll never know the difference, but I would recommend common grounds. it may just be as simple as powering your dvd drive seperately, or you may need to remove another accessory or two, or just power a usb hub seperate from the opus, depends on how overloaded your opus is...
I think that power supply will need to be connected to a motherboard to function correctly, or at least see what it should from the mb & I don't know how you could piggieback it with the opus, although I intend on looking into doing something just like this, it shouldn't be to difficult, would just need a digram showing control & feedback wireing I guess....the easiest thing to do would be to just get a 12v supply triggered by a relay from the opus 12v output...
if you DID want to damage your opus, constanly overloading it to the point of shutdown is a good way to do it, I did some research on using multiple power supplies & the info. is readily available, I have seen diode isolation & resistors to stop the two from trying to "charge" each other & i have even seen one setup using mosfets that seems like the best way to do it.check here to see this, but again, THE EASIEST thing to do is get a 12v power supply & power it off a relay controlled by the opus, you can do this probably with stuff you might have laying around & even if you had to buy stuff it should be pretty inexpensive, I'll bet it's cheaper than replacing your opus