Detecting Second Monitor that does not exist

I have an Inspiron N5110 that is about 2 years old. The hard drive was replaced in January this year after being damaged. For the past couple of months I have had a problem with a second monitor being detected when the mains power cable is connected. There has never been a second monitor connected to this laptop. The screen resolution changes and if you have the Display panel open you can see it switch between single monitor and dual monitor. Every time it does this the screen goes black and then comes back up, the mouse pointer resets to the middle of the screen and it makes the noise you would expect when something is being plugged in. All of this does not happen when running on battery power. It happens most when the battery power is almost full or almost empty and the screen can change anywhere up to every 1-2 seconds at this time. When the charge is more towards the middle of the battery it happens a lot less often, maybe only every couple of minutes. It is almost to the point where the laptop is unusable if it is charging. We have checked the display drivers and reinstalled them just in case it was that but it didn't seem to make any difference. Please help, it is driving me crazy!!!