Usually the issue is them getting rusted into place and breaking on removal. Well, I have the opposite problem. Properly installed and torqued studs with nuts are loosening themselves... The nut is staying in the position it was tightened to, but I've had the stud itself walk out multiple times now. It tightens in nicely and will be fine for several weeks. Then all of a sudden it has walked out (I'll find it hanging on by just a couple threads, or like last night where it was just sitting in there all the way out of the threads!).
Book seems to suggest the clean & dry plus torque is appropriate, but that's obviously not working. Are there any high enough temperature, but relatively low removal force (blue at best) threadlockers that can tolerate the heat that an exhaust manifold stud will see?