I have a core 2 quad q8400 and my idle is about 50°C, it's already high, but when i turn on Prime for example for testing the temperature goes to 100° in one of the cores and 90° in the others in 30 seconds. After stopping Prime the temperature falls to 60 in less than one minute too. The sensor might be broken or the cpu is really burning? I heard that generally the temperature in a cpu doesn't change so abruptely. I have currently the intel cooler that came with it, which cooler should i buy? I heard CM hyper 212+ is quite good. My desktop isn't very big, Noctuas I know are very good but they are designed for bigger desktops right?
The initial batch of second generation core 2 processors are known for having sticky thermal diodes that are not reliable at low temperatures. However they aren't too bad at higher temperatures. I'm not entirely certain if the Q8400s experienced this problem though. I would get a second opinion from HWInfo64 because your motherboard might have a few more thermometers which can be used to gauge temperatures.