Ouch! 78C?!? Even on overclocked systems that is dangerous! Very very hot! I would turn the computer off, check the CPU fan, dust it. Hold the fins on the fan and then blow it with one of those air duster thingys. What is probably happening is something called THERMAL THROTTLING. The computer does this by ITSELF. When the computer gets really hot, i.e. 78C, the computer will automatically down clock your CPU, maybe even to 1.4 GHz or less!
So I think this is what is happening. You turn the game on, you are all fine and dandy. After a few minutes, the CPU gets hot, and gets hot fast! The computer doesn't think this is safe so it automatically downclocks the CPU and that is where you are getting FPS such as 20fps. This is actually a good thing, you might've burned the CPU or at least damaged it if the Thermal Throttling wasn't there.
Here, can you see what the temp of the CPU is at IDLE. To get to idle, turn off all programs except for the CPU temp measuring thing, don't touch it for 30 minutes, then write down the temperature. Then wait five more minutes, keep the temp measuring thing on, turn your game on for a few seconds, i.e. start a mission or whatever, just get past the menu, and then immediately minimize the game and write down the temp and if it changes at all. Make sure to write the changes in temperature.
To finally make sure that this is Thermal Throttling, though I'm pretty sure, we are going to check the clock speed. You downloaded an app that showed the CPU temperature, does that utility by any chance show clock speed? If so, get to idle, check the cpu SPEED (not temp this time) and then launch the game, into some mission, minimize as quickly as you can and watch the clock speed.
So clean the fan, make sure the fan is actually PLUGGED INTO THE MOTHERBOARD, and reply back!