The i5-2500k max recommended temp is 72.6 degrees celsius. This is for the CPU only, not the "cores.' Intel says not to worry about "core" temperature, only CPU, which you would check from the BIOS.
If it's CPU at 70, I would recommend an aftermarket cooler, and organizing cables for optimum airflow (as seen above--it really does make a difference if you can make a wind-tunnel effect). Or, you could try reapplying the heatsink paste with Arctic Silver 5.
You should be able to gain control of your CPU fan speed from the BIOS, try increasing it if you can and see how the results are.