I've got an idea. Post enough information so we can actually help you. Full system specs, temps, stock HS/FAN? Aftermarket? overclocking settings? Voltages? FSB? Multiplier? Motherboard? The questions go on and on.
Restarts are usually temp related so if I had to guess you using the stock HS/Fan to try to OC. Get a good aftermarket cooler and repeat the process.
These settings worked for me on a BIOSTAR G41-M7 with a CoolerMaster and 2GB of 667 RAM, as you can see I work at 3.00GHz without any issues, the temperatures are below 60ºC for the micro and below 40ºC for the board