problems on resume from hibernation / standby

When resuming from standby, my computer crashes instead of showing the
login-screen.
sometimes everything works right, mostly not.

I tried everything from changing graphics card drivers to disabling
auto-power off for the monitor and some of the changes to the registry
suggested in this group as well as a clean reinstall.

Also, when resuming from standby, sometimes I can logon correctly, but
then the computer suddenly reboots without warning or bsod.

if anyone got an idea what might be causing this, plz state it.
i need hibernation badly, as system startup is quite slow on a notebook.

thx in advance
markus