27" iMac as a display, wierd connectivity

Hey!

So I bought a PC that I want to connect to my iMac via a MiniDP to MiniDP connection. However, it rarely works directly. Instead, when I power up my iMac, and the PC thereafter, nothing happends. I wait for the PC to enter Windows, press the correct command for display mode, and the screen turns black, and back to OS X. This goes on until I try some wierd combination of starting the iMac/PC at different times and pressing the display command at random times. Really frustration, since it should work.

Advice is very appriciated.

Anyone with advice?

Using a 27-inch iMac (Late 2009 or Mid 2010) as an external display

Read this and see if there's anything you overlooked.
Personally I don't see why it would only work when one starts to randomly press the display command, or start both machines in a random order, so there should be a solution to it.

SM