On my Acer laptop, apparent dimming then brightening was caused by a setting called "Display Power Saving Technology", a feature of the Intel Graphic driver. This was fine and sorted when I disabled it. HOWEVER, the setting was automatically enabled again whenever I unplug/plug my computer or let it shut down or hibernate. This frustrated me immensely. But I discovered the solution: You must delete Acer's own program called "Acer Battery Management". Do that and the setting won't be constantly changed by that horrible garbage.
I would need more information in regards to your system configuration:
Which version of Windows do you have?
Also, the exact processor model and the graphics controller:
Click the Windows icon, and then right-click Computer .
Select Properties .
Find System Type on the Properties window, and then note the processor model.