Windows 7 - My restart problem.

I have a AMD Phenom II X2 550 3.10ghz, for some reason it is not allowing my pc to restart, when i choose and click on restart my pc closes down and it turns back on but the monitor stays off and i have to turn it off by holding the power button. I was in the bios settings overclocking it to 3.4ghz and i saved the settings and closed bios and the pc restarted ok, i then tested it by restarting a few times and it was successful, so i went back into bios and put the settings back to normal and closed bios and guess what the same problem.

To fix this, all i done was go back into bios and changed the multiplier and voltage option from auto to x15.5 3.10ghz (which is the default multiplier and speed etc) and closed bios and restarted happily without any more hassle. When i have multiplier and voltage set to auto the default is x15.5 3.10ghz.

Any ideas ppl?????/

I now understand why this happened.