Found this thread via Google just now. I have a 4 month-old(new) i5-2500K, not overclocked, on an MSI P67A-GD65. Cooler is a Noctua NH-U12P SE with two fans. Case is an Antec Three Hundred in standard layout; no extra fans added.
For (about 4) months Core Temp showed all things normal. However, since a few days now, it show first one, then a second, and then even all four cores going up to 98% Celcius, all in idle mode.
I installed Real Temp (ver3.60), and it reports nothing out of the ordinary while idle: 26-28° C and speed in idle is down to about 1.6 GHz.
Three people for whom "Core Temp" is behaving in the same way? I think we can be reassured it is not our "2500K"...
Perhaps a Windows update messed up "Core Temp"?
Good catch Dassie, Thanks.