Brightness not working correctly on Dell 6400

I saw some other threads on this topic in the past, but have not been able to find a solution.

When hitting the Fn key and the up or down arrow, the brightness moves in 2 or 3 steps at a time. The only way I could get this function usable is to set acpi=off at boot. this is not the solution that I want.

Has anyone an idea as to what the problem is and how to fix it?