Viewer doesn't Run! It's Always Black!

I tried Final Cut Pro X on 3 different Mac: on a Macbook, on a 27'' iMac and on a 21'' iMac.

Macbook and 27'' iMac it's all ok...

on the 21'' iMac (early 2011) i can't use FCPX because the viewer is always black, and every time I press Play, this mac becomes too slow and only after 20-30 second it "plays" the project (even if skimming, i can't see anything... just black!!).

i tried to "Revert to Original Layout" clicking on menu Window, and to delete the file called com.apple.Finalcut.plist in Library Folder, and even to reinstall the program.... but i can't solve the problem.

help!