[Precision M4600] No dim control

Hi,

I'm currently running Windows 8.1 on my Precision M4600. I sometimes get dim control, sometimes not. All Dell softwares are installed, including Dell Enhancement Pack and the keyboard thing.

I've found a lot on the Internet, but they all have a nVidia graphic card while I have an AMD one (FirePro M5950 = HD6740)
What's odd is that it is completly random, sometimes it works, sometimes it doesn't, sometimes a reboot fixes it, sometimes it doesn't...

It really is anoying, so, if anyone has an idea...

Mika.

Ps : My service tag is <ADMIN NOTE: Service tag removed per privacy policy>