The word monitor means to look at, so when you ask for a thermal monitor, you are only asking for something that reads the temperature. The process you describe to try and limit the temperature is called throttling. It is the last ditch effort to prevent overheating, and is supposed to be done automatically by the bios. If the system is properly designed, it should not even be needed since when the fan is running at full speed, it should not overheat under full load. You should make sure that your fan is working, and not clogged with dust. Aside from that, use the frequency scaling applet to limit the processor speed, which is better than throttling.