You need to clean it. If it is shutting down, it means it needs to be cleaned and no amount of external fans are going to help. When your CPU heats up, the heatsink is designed to draw that heat out, and then the heat sink fan blows the hot air out. When that gets clogged, the hot air stays in on the heatsink and the heatsink can't cool the CPU anymore. That is what is happening here. Adding external fans, a cooling rack, etc. will cool the surface of the machine but will not cool the CPU.
Even if they did work, you only get about 4-5 degrees celcuius out of the best of them; if you are suffering from thermal shutdown 5 degrees isn't going to be sufficient.