Have you double/triple checked the seating of the CPU and thermal paste ??
Well, I had to re-do the screws around the heatsink a few times, sometimes one of them doesn't catch. When all four bottom out, though, it's attached correctly. As far as thermal paste goes, I just threw a dap of MX-4 onto the center of the CPU, and when I took it off, it seemed adequate coverage. I didn't clean the prior paste off of the heatsink, but visually, it didn't seem like there was much of any on there.
Anyways, I cleaned off the CPU, grabbed a 95W 1600 heatsink out of a box, and cleaned the thermal paste off of it (there was a weird off-color stain in the paste and copper core, like something liquid had dripped in, but the box was mostly sealed), and then applied fresh MX-4 to the CPU (half pea), and then attached the heatsink, all four screws bottomed-out.
So, right now, mining, temps on CPU according to Ryzen Master, are 86C. PPT is 87% of 88W. Clock speed is 3.88Ghz.
Edit: I switched from mining, which makes the entire machine especially Firefox Nightly to become unresponsive (not responding for like 2 minutes, to a click), to running BOINC, PrimeGrid (since I couldn't get WCG to give me tasks), and running 90% of CPUs (10 threads) of CPU crunching, and 2 threads (2 RX 570 8GB dGPUs) crunching GPU tasks, my Task Manager CPU graphs are all pretty-much filled, and Ryzen Master was reporting, briefly, temps slightly ABOVE 95C, when it lists 95C as "max". :|
Also, HWMonitor reports clock speeds of 3.75Ghz, at those temps and loads. This is with PBO enabled.