Thanks for the insight, Aiboh. I'll try to do some testing when I have time and report back... for science.
Update: I am somewhat more certain that the PSU is tripping its over-temperature protection. I removed the side dust filter and put a large external fan blowing at the PSU intake, and then ran Unigine Heaven for over an hour without issue. Ambient temperatures were roughly 25°C.
When I turned the fan off, CPU and GPU temperatures rose slightly but were still okay. The PSU fan started to turn on occasionally when the FPS counter spiked high, and it was quite loud for a few seconds, then shut off. Eventually, after about 15 minutes the system abruptly rebooted. Furthermore, it restarted over and over again during bootup while the BIOS was doing its thing, until I turned the external fan back on.
I considered getting a infrared temperature gun to try to check the internal temperature, but I think I'll just save myself the time and money and just return the SX700 to newegg. I'm really disappointed that I waited so long for this PSU and it's not working out for me.
Maybe Silverstone's next attempt at a semi-passive fan controller will finally get things right, but for now I think I'll try a different brand for once.
Based on your experience with the Lian-Li PE 750, do you know if it is also hardwired to ignore temperature until a load theshold is reached? Or would I be better off getting a Corsair SF600?