Thank you very much for the reply.
Under heavy usage the fan turns on. So it is not broken. Might be blocked.
I tried to increase the cooling by putting the laptop on a metal plate to cool it. However the freezing persists.
I tried to open up the laptop before, without success. How I miss my desktop sometimes :-)
To be a bit more precise. The laptop unfreezes when I push slightly on the casing to the left of the touchpad. So I push on the harddrive and the memory. Sounds funny, but googling a bit I am not alone with that problem.