I'm using a Tuniq Tower which uses 4 thumb screws into a base plate on the bottom side of the motherboard. I generally tighten until all 4 screws have bottomed out at the end of their threads but now I have serious doubts if this is too much pressure and causing some of the issues we're seeing.
The H bracket that rests on the top side of the heatsink base plate gets bent after a good snugging. I've also read the Intel docs and know how specific they are about contact pressure so there is likely a good reason for that. I might need to try and find some hard, non-compressible rubber to mount between the bracket and the motherboard to prevent excessive force.
While running Prime, I'm going to monitor the temps and then back off the screws one turn at a time to reduce the load on the IHS and wait to see what happens.