@slender thx for the answer after i saw the youtube video about this bug i can tell you that this isnt the Problem i have. The screen "glitch" i dont noticed. The Problem is only when my N900 is locked by the slider or over the menue.

if it is locked and i get a call i see the Call for a second but before i can hit the answer button the screen turns black again. i need to use the slider again and then i have slide the screenbutton to unlock the device. Until this time i lost the Call.