Mac pro mid 2010 - screen suddenly turns black

hi everybody,

since today my Mac pro (snow leopard) does the following:

Sometimes the screen suddenly turns black. Only the Cursor can be seen (sometimes not even that).

When I reboot the system everything seems fine, but after a few minutes the same problem occurs.

Unfortunately I'm not able to copy the specifications of my system because the black screen is faster than I am after a reboot.

I would be glad if anybody knew what I can do about it.

Thanks guys! Chris