For starters I would first try cleaning off the dust build up on the graphics card heat sink and also think about replacing the thermo grease between the heat sink & GPU chip/s with a good quality one, and don't forget to clean fan blades as well.
If the rest of the system is dusty I would also clean it out as well.
Use a soft brush and can'ed air and use proper ESD practices with a good mat & wrist strap.
You may have a bad sensor if the problem persists. Slowing down the fan forcefully may not be a good idea as your graphics card could be over heating.
You could try getting some software to monitor the system temperature sensors to see whats happening.