I'm having some interesting temperatures on a Core i7 4790k I'm working with. I noticed that idle temps were ~80-90c and found that the CPU fan seized due to a tightly wrapped cable around the fan.
The fan ran pretty rough and uneven after I fixed the tight cable, so I replaced the heatsink and fan with a Coolermaster Hyper 212 EVO (which I've had great success with in the past) and applied Arctic Silver 5.
The idle temps are down to ~30C now, but under a Prime95 stress test (max heat) they instantly go to ~100C. Is it possible that something is damaged in the CPU? The stock fan was probably seized for many months.