I would start with memtest. It is distributed in many live cds.
Thanks! Memtest passed w/o errors. I then booted live VL6-Light and could not
induce a freeze. Went into my old VL5.8 box no freeze either.
Back to my working box und made the freeze since by now I know how to.
Logged-in as root opened X with gnome and no freeze.
Now the no freeze tests have one thing in common X is not started with nvidia.
Changed therefore now to vesa and sofar could not make a freeze. Will keep
on watching it.