|I wouldn't cut the speed senor wire, that could stop the computer fro working at all. The BIOS will think that the fan has stopped altogether and cut the computer off as soon as it gets started.|
You need to find out what is causing th overheating in the first place. What temperatures are you getting?
When you fitted the heat sink to the CPU did you use the correct amount of thermal paste between heat sink and CPU.
Are you sure it is not the graphics GPU that is overheating? Shutdown during gaming would indicate that as a possible problem.