Ok, so the lines are actually moving. And it is more than 2. I bet they are not perfectly straight either. In this case it means that the monitor is dying, or that it needs to be readjusted. Either way it would be best to replace it.
I have seen that pattern many times, I forget what the circuit is called, but often a capacitor starts getting leaky.
Get a new monitor. And Dell monitors are not bad, they are made by major brand names but get the dell sticker. Often they are the low end, not the best.
I have had mostly good experinces with Dell monitors, but monitors in general go bad randomly.