Use Realtemp or Coretemp, and trust the temperature reported in BIOS - at least for now lol.
Yes, check and re-seat the heatsink. It would be best to remove the mobo if you need to in order to see that each of the four "legs" has passed through the mobo and each leg's split-pins are locked securely underneath it by the pin you pushed through the center. (Assuming you used the stock cooler, as you didn't specify.)
As for temps, stability . . . its all a matter of degrees