kernel_task CPU 108%

Hi. I have problem with my Macbook Unibody late 2009. Last week I turned on my MB and heard my fan at full speed. In activity monitor, there is kernel_task with 108% usage of CPU. Restart - nothing. I reset PRAM and NVRAM. Then I run HW test (the long one), but with no problems. So I reinstall system with disc erasing, but the problem is still here. Sometimes kernel_task takes 108% immediatelly, sometimes after eg 10 mins. I install app for showing temperature and all temps are relevant. I don't know what to do. I tried to change RAM, didn't work. Where is the problem?

HW test shows nothing -> no HW problem
Complete reinstall -> no SW problem

Please, help me, I'm hopeless...

PS Sorry for my English, I'm from the Czech rep. And thank you for any help...