Bitcoin QT using CPU

I've a fairly decent laptop, with good CPU, 4Go of ram and a SSD harddrive. Nevertheless, when I run the bitcoin client (0.4 or 0.5), the whole computer freezes for several minutes then is usable for a few minutes then freezes again, etc.

I've monitored: CPU is low (bitcoin uses 10%), memory is low (bitcoin uses 3%), network isnormal (bitcoin uses 50/100Kb/s).