In your control panel, under performance and management, you have the power options. If you go through power options you should be able to locate the VAIO power management tab, where you will see the CPU fan speed level. Check to see if that level is set to the maximum, if not then it should be.
What i meant when i asked you if your computer only restarts during hibernation , is that , does it restart itself normally ? Or is it just while hibernation ?