Screen brightness goes crazy

I'm trying ubuntu 9.10 beta on a MSI Wind U90 netbook, and I daily check for updates.

At start-up, my screen's brightness starts to crazily go up and down. This also happens every time I try to manually increase the brightness through the keyboard (decreasing it seems to be ok).
More precisely, the brightness goes up and down by only one step, and, while this is happening, I can still change it with the keyboard (for example: if the brightness is hopping between 3 and 4, and I press the key to increase it, it will start hopping between 4 and 5).

The only way to stop it seems to be invoking gksu (I usually use 'gksu nautilus' or use the cpufreq applet).
Anyhow, I can't exactly figure out if there's a relationship and which it is. Sometimes, it immediatly calms down when the password popup appears. Other times, I need to insert my password. Rarely, it won't work at all.