I just want to announce that starting grml (old 2008.11 release) which loads the nvidiafb driver and then start the xserver repaired my graphic card. It was big luck, that I tryed to start the X server again in grml after hours of working with only external monitor.
Now everything seems to run fine again. But I need to do more checks.
I think it was *not* a heating problem but, because of a bug in 260.19.37 with my xserver, my graphic card was misconfigured so that it does not work completey until it was reseted by nvidiafb or some graphic RAM 'forgot' some wrong values (which needs longer than 6 hours without power as I could detect weak, but well-known icons on the screen during the time the screens showed only strange patterns).
I don't know if this possible or if there are other reasons for the resurrection of my laptop monitor. But now I am happy, that it works again