I´m having a similar problem with a FX6100 running Win7 64Bit.
Randomly shuts down without a BSOD. Occasionaly reboots by itself but most times I have to retart it myself. Also occasionally hangs on the POST screen after a re-start but starts okay on a second attempt.
Temps all seem to be okay and I can´t detect any hot spots within the case, 4 fans blowing in, one bottom front over the drives, one top rear and two on the side. 2 fans extracting on the top.
Ran a program that could report the system voltages and they were all low. The 460W PSU may be on the limit for this system so I have ordered a 600W model.
I´ll report it if this fixes it or not.