If you have checked your power savings mode and it is set correctly (each OS is slightly different but similar) and it still happens even when its plugged in, then most likely your LCD power inverter is failing. If it was your back light, that generally just goes out completely and won't come off and on. Either way not to expensive or difficult to replace. The inverter on the most Dell laptops is soldered on to the LCD panel as a ground. Removing it is the hardest part of the whole thing.
And yes I am a Dell Certified Tech, just giving some general free advise.