Yeah, first of all 80C is really hot. Secondly, your stability is not directly tied to your heat. A lot of heat can be one of the causes of instability, but the formula for overclocking is not simply "keep the heat down and it will stay stable." I've had blue screens and instability problems at temperatures below 50C. You can overclock a component and start to get instability long before you reach high temps. Stability is mostly a matter of a) is it possible for this part to run at this speed, b) does it have the juice to do so, and b) will running the required juice through this part cause instability or overheating at that speed.
Normally you slowly increase speed and when it begins to approach instability you check your temps, and if your temps are still low you increase your voltage to see if that will stabilize the higher speed. You then retest to see if the increase in voltage stabilizes the higher speed while carefully watching your temperatures. 80C is ridiculously hot though. You may have simple topped out.
To be perfectly honest, asking why overclocking up to 80C and then running one of the worst stress tests imaginable causes blue screens almost sounds like a troll post. That's what overclocking and stress testing does.