lots of screen freezes with LXDE

Hello everybody,
I’m new to opensuse, coming from Lubuntu. (At first glance, Lubuntu seems to me lighter, with 71 Mb of RAM at start, against 125 with Opensuse+LXDE :frowning: ).
But what interest me here is the rolling “Tumbleweed”.
So, i’m using 11.4 + repositories Tumbleweed + LXDE and my screen freezes partially very often (for example, when I move windows, go up and down in Firefox or Midori, …).
My computer is a laptop Toshiba Satellite L10, 512 Mb of RAM and 1,6Ghz.
Can you help me to solve this?
Thanks in advance.