Originally Posted by solidbarton
I should also note that my Antec is probably going on 4+ years old, with moderate use almost every day.
Although your old Antec PSU isn't underpowered for your PC, it could have very well been an aged PSU that could no longer handle the load causing the reboots. Suspect some swollen caps, etc.
If the PSU was actually overloaded, it should have shut down. But a bad PSU can certainly cause the PC to reboot.