The Athlon X4 640 has a max temp threshold of 71°C
But it depends on the motherboard, doesn't it?
You are showing 78°C tmpin0 that is past the thermal threshold! As far I have read that is the system temp which is still high, tmpin2 should be the north bridge.
The relation between the tmpin0 and core temps is unbelievable but cores to tmpin1 is believable!
I don't have a solution for you but this info from my experience may help. On all 3 Athlon machines I have built the core temps have never been accurate ie the machine I am typing this on says core temps are 8C when the room temp is 19C. The CPU temps though always seemed accurate as far as I could tell and where called CPUTIN in hardware monitor. In hardware monitor I get 3 temps apart from the cores which are SYSTIN I assume the north bridge temp CPUTIN the CPU and AUXTIN that I am not sure about it the Athlon systems I have built it seems to run roughly 10C higher than the CPU in the Phenom system I built it reads 80-90C when the system is idle but drops to around 40C under load so I decided its a faulty sensor or a faulty reading.
Case flow,case flow. Your Scythe Katana 2 came with a four way mounting bracket ? If so mount it horizontally pulling air from the front toward the rear exhaust fan. You may need to move your memory out to the black slots so the HSF has clearance. As it is your not getting very much fresh air to the cpu pulling off the top of your gpu, and unless you have exhaust fans in the top of your case the warm exhaust air is being blown back into the case so to speak..