It's probably not your cooler.
I've had an FX 8350 at 4.5GHz on that cooler with 1 PWM fan and it kept it under 60c during stress testing with all 8 cores at 100% load. Something like a Noctua NHD14 or Phanteks TC14PE will give you 3-4c lower temps, but not much more than that.
Hyper 212 EVO does NOT benefit from a second fan. You will only see a 1-2c difference. Waste of money.
As for why it is rebooting, there's not enough info here to make an accurate diagnosis. Include detailed system specs and settings so we can help find out what is causing it. And even then, it may not even be a hardware issue, especially if your voltages and temps are fine. And I've seen some recent game patches, Flash updates, steam patches and driver updates cause crashes in some games.