an update:
i've found this information:
Documentation/sysctl/kernel.txt
==============================================================
tainted:
Non-zero if the kernel has been tainted. Numeric values, which
can be ORed together:
1 - A module with a non-GPL license has been loaded, this
includes modules with no license.
Set by modutils >= 2.4.9 and module-init-tools.
4096 - An out-of-tree module has been loaded.
==============================================================
maybe the non-GPL module was the nvidia module.
I dont know what 'out-of-tree module' means.
anyway, uptime is 14hrs and still no problems :)
>From: "asronchetti@libero.it" <asronchetti@libero.it>
>Date: Mon, 6 Aug 2012 15:54:40 +0200 (CEST)
>
>and kernel was tainted:
>
>kernel.tainted = 4097