I just got an i7 8700k from hardware swap and the 3rd core is running way hotter than the rest. Its sitting at 68 degress celsius, while the rest are in the 30s. I've reapplied the thermal paste and it didn't change.

Try to shutdown your system and leave it for a while. Hopefully it will get normal.

@cukote. That's a big delta T. But here's a few things.

  1. Not all temp apps work right.
  2. Check it out in apps like Speccy and CPUz. (see google)

If it still that far off, consider having the CPU heatsink checked out. It could have lifted off on a corner.