does Dell have any power management software like Acer has it's Empowering? The dimming and stuff can be a hardware fault, but it can also be a software-caused problem. Reinstallation of this software and/or a fresh installation might help.

Also, what about if you go into the BIOS management screen - does it dim out there as well (because then it might be a hardware fault).

And does it happen when the laptop is connected to the power or when it's sitting on it's battery energy only?