More explanation on the PSU(Power Supply Unit) : It converts the power from the wall plug to the lower voltage that the components in your computer need. If it has a problem, it's making interference in the power or the voltage gets wrong and your computer works bad. Sometimes people call it a transformer.
Freezing is just an expression, it just means that there's no more activity. Too much heat could cause it to "freeze".
If restarting changes something, I would look, like the others told, the Power Supply and the Hard Disc Drive. When I had a similar problems, I had to update the BIOS and change it's settings.
BIOS : The "motherboard screen" is what you see from it. The BIOS is a very small piece of software on your motherboard allowing your computer to boot correctly and is also used to keep the hour and set the motherboard speed.