I've experienced this with my 2009 model recently. I explored all available options short of cutting holes into the bottom of the Macbook to channel more air through the vent. In this instance it turned out that Apple's thermal paste application was sloppy at best. This caused major overheating issues for me and would cause kernel panics whoever the Macbook reached temperatures that exceeded the maximum tolerable allowance. Fortunately the Macbook was designed to shutdown without causing damage.
It was necessary for me to open up the Macbook and remove the board and the cables very gently and apply thermal paste to the processor and the GPU chip.
Please see the guide below. After I completed the task (very carefully and gently) my Macbook has been running without the need to include a third party software to regulate the fan speed.
MacBook Unibody Model A1278 Heat Sink Replacement
The guide can be used to clean out the existing thermal paste and reapply it with a better quality and at the proper quantity. I would also use this opportunity to completely remove any dust from within the unit, including but not limited to the fan.
Please let me know how this turns out. I would love to hear the end result.