Thanks a lot! This should be a possible work-around for my problem. I'll try it as soon as possible.
Nevertheless I still wonder why the fan speed control gets stuck from time to time. This is not what the whole fan control thing was inteded for. I run the most recent video card BIOS which is offered by Gainward. Yesterday, after submitting my posting I upgraded the drivers from 280.13 to 285.05.09. Perhaps this fixes it. But I don't even know in how far the driver is involved in the fan control...