Strange behaviour in my laptop. Please help

Hello,

I found a strange problem in my laptop. ( Make is DELL and model is Studio ). When i connect the external power to my laptop it automatically types the + key in regular intervals of time.
One main problem is with Google chrome. It automatically switches to full screen. If i press F11 it returns back to normal screen. This happens in regular intervals of time.

But my laptop is behaving normally if i just disconnect the external power and running it on battery. Can any one of you have some idea about this? If you know any other forum where i can get answer for this then please tell me.