Console freezes

[Erix]

I've installed my gentoo linux system starting from step 1 with a
minimal installation disk and i managed to solve almost evry issue, but
i still get this problem:
*sometimes* when i switch between consoles with alt+fn or (more often)
between console and X the screen freezes and goes blank when switching
to/from X or freezes on the last console when switching between
consoles. This is quite annoying because i can't predict when it will
freeze, in fact sometimes it never freezes during an entire session.
Even if the screen freezes the activity of the processes already
started doesn't stop (internet and disk leds don't stop blinking) but i
can't issue new "blind" commands, for example i tried a shutdown but
nothing happened.

My kernel is 2.6.11-r5 (but the problem was there also with r4)
I have a nvidia geforce 6800 ultra, but the problem was there also
before installing the nvidia module
I have framebuffer compiled into the kernel

Can anybody help me please?

Thanks in advance