PC configuration: XPS 8500 with Radeon HD 7870  connected to a U2412monitor (1920x1200) using a displayport cable and W7. I am using the application "power napp" for the monitor.  Normally the XPS/monitor are working as expected. After a certain time in inactivity, the XPS and the monitor enter into sleep mode(correct behaviour). When I press any key the XPS and monitor "wake up" as expected(correct behaviour). Sometimes (5% of the time) the monitor wakes up with a lower resolution and I have to reconfigure the resolution to 1920x1200.

I am using the application "power napp" (U2412) and I am using "displayport" cable to connect PC/monitor, could this problem be caused by the use of displayport? Any other suggestions?