Turn off the laptop, Use a compressed air can the blow through the vent. You may have a lot of dust bunny fly out. After you clean your laptop should be fine. Toshiba is known to get overheat, base on my experience.
Jiss, have you noticed anything wrong with the power supply for the laptop? What i mean is, are there any visible cuts/wires showing on the power cord? How about the part that connects to your laptop (the power plug) look? DOes it look bent or damaged?
What kind of laptop is it?
How old is it?
Is it second hand?
Have you tried looking in the system tools - event viewer - Windows logs - system or application to see if anything comes up at the time of restart?