I initially thought it was how hard or certain areas of the button you pressed but it seems not. If you power yours on and off in quick succession does it respond to each request correctly?
Are you seeing this issue:
http://en.community.dell.com/support-forums/peripherals/f/3529/p/19354040/19779295.aspx
Suspect the kettle lead has an issue as it makes a static noise when wiggled around. Same on a different monitor. I think the power button also has issues though as if it was purely the kettle lead I'd have thought the image would flicker.
Ah okay - thanksI think it will likley just be an issue with the monitor as all the monitors I have put power leads in do that noise.