Well.. to have a faulty core, is to have blue screens. To have a faulty sensor is different, and perhaps what your experiencing. Since your using a 6400+ BE chip, its not clear how the temps are determined.
Having the HS tight is one thing, but even pressure is another. I'm guessing your using a tower case, so the Zalman would have more pressure on one side of the CPU. Have you tried laying it on its side so the weight would be more even?
In my experience with AS5, I'm sorry to say the cure time didin't really have any affect in decreasing the temps for any of my past applicatons. The only profound effect I saw was when my ambient temps were cooler. So in my mind, I kinda wonder if people over look that, and mistaken the cure time made the CPU temp cooler. I generally use MX-2 since it doesn't have a cure time, and is supposedly good for 7 years when applied. :lol: . o O (don't think I'll have the HS on that long)
Since you mentioned the stress test causes a shut down, but is cooler with the case side off, I can only assume that your airflow is constricted, and perhaps your ambient temps may also be high.