Suspend sometimes hangs the whole system

Hello,

Normally suspend/resume is working fine on my Lenovo Thinkpad T61. But
sometimes, when I press the power button to suspend, the whole system
hangs and does not suspend. In X11 the mouse cursor goes away and no
mouse action is possible. The graphics screen is staying there and it
does not switch to console as on a normal suspend. Also manually
switching to a console screen is not possible. Even the ssh daemon does
not respond when I try to login thhroug ssh from some other pc. Looks
like sshd is already sleeping. The only way to get it back to life is a
hard reset.

How can I investigate this problem further?

-- 
Mit besten Grüßen
Jochen Fahrner