screen turns black after 2 seconds until i move mouse or type

[rubo77]

i connect from my Ubuntu 11.4 to a remote Kubuntu 11.10 mashine
usually it works fine, but today i always have this problem:

after only 2 seconds of inactivity the screen turns black.
when i move the mouse in TV or type a key it comes back for another 2 secondes.

the only way i canwork is, to move the mouse all the time or click somewhere and don't release the mouse button.