Coretemp only reads the CPU register see
directly access the data from within the Intel CPU register that holds the digital thermal sensors (DTS ) for the respective core. The absolute temp is purely TjMax (105C on 8400) - DTS value. I have verified this myself using the method described. (cpu-z dump). Now that basically is so simple it puts the onus firmly with the CPU. This leaves us with the following posabilities.
1) TjMax is overstated.
2) DTS is understated.
3) Or the cores are running hot, in the same way thw Q6600 B3 chips did.
In any of these scenarios the purchased product is at fault.
luck