Black area in monitor after starting up computer

Whenever I startup my pc a part of my Dell U2713HM monitor is 'black'. When I disconnect my monitor from the power and put the plug back in, the problem is gone and the screen is normal again. This is something which happens a month now. Sometimes I have to unplug the monitor once, but more often I have to unplug it from the power more often to get ridd of the problem.

Anybody who has a suggestion?