FLASHING LAPTOP SCREEN ON PLUGGING IN POWER CABLE

Hi,

I have a Dell Laptop , and it has been working intermitently, sometimes not coming on at all. Then we tried switching it on with out the power cabl and it came on fine. Then when we plugged in the power cable the LCD screen started flashing dull and then bright as if you were taking the cable in and out.

What could be the cause of this problem.

Any ideas would be appreciated.

Regards

Helen