Alerady tested the memory, and i acused no problems. :-) It was more or less reproducible, just had to wait a few seconds/minutes and it lock up. Changing back to the open source driver "nv" it worked just fine. Also it didnīt lock anymore when I seted the nvagp to 1 stating that agp from nvidia should be used, before was setted to 0 (no agp).
Now with AGP set to use NVidia driver it just locks up when changing the console from text to X using alt-F7 key after doing an ctrl-alt-F1. It does not lock up every time, just 1 out of 3 or 4 times, once again only with the nvidia driver, it never happend with the nv driver (witch I user for a few days). Since I have a log running in the tty9 screen I do this operation quite commonly.