I don't think this is your problem if you've already taken off the HSF a few times, but I'll add it anyway. I had 79 degrees at 100% load with my 1700+ because the HSF was mounted backwards... after turning it round and mounting it a bit more securely, I was down to 39. Less with the case off 😛
I dunno, the way you got a lower temperature by putting on a more powerful fan sounds similar to what I had. Even with the HSF on completely wrong, I could get down in the low 50s with a desktop fan blowing across the CPU. It sounds like it's not on correctly, because that was what happened to me 😛
Probably completely wrong but I just thought I'd add it. I never had any instability at those huge temperatures, but running something like SETI just isn't as fun when you know your cpu is slowly burning up 🙁