I found the new driver 5336 today and installed it. I thought the problem would perhaps disappear with it, but it didn't. Only this time, my machine totally froze after ca. 40s of the screensaver. Not even the Magic SysRq didn't respond.
After that I included the line
Option "NvAGP" "2"
to force the use of AGPGART, and till now, there's no problem... Hope it stays that way.
Thanks for your help!