The computer will turn itself off or enter the suspend to disk mode automatically within a few minutes after a cold start.
These systems have a thermal protection circuit which will shut down or suspend the system if the system overheats. This is designed as a protection. The typical cause of this condition is poor air flow in the system. The system has two cooling fans, one on the bottom of the computer and one on the side (MP975 only has one on the bottom). The vents on the bottom and sides of the computer must be free and not have restricted air flow. You should not operate the computer in an enclosed case or on a soft surface which will conform to the shape of the bottom of the computer and block off the vents. Whenever possible, the legs in the rear of the computer should be extended during use.
The fans in the computer were designed to turn on while the computer start up so that you can see that they are operating properly (except MP975). After the initial start the fans will stop and then follow a thermostat to cycle on and off as necessary. If the fans become clogged with dust or otherwise fail, the system will overheat and shutdown. You should check and blow out any dirt from these fans. With the computer turned off, you can remove the door covering the fan and check that it is clean and spins freely. If a fan is not running or spinning very slow, it should be replaced.
Another possible cause of having the computer turn off is running from a very low battery. Sometimes if there is a problem with the AC adaptor (not plugged in or plugged into a dead outlet) the system will run off the battery without the user's knowledge. If the system is turned back on after the battery runs out, it will run for a short time and turn off suddenly. When the battery is low, it can die before the system has a chance to sound the warning beeps. To be sure that you are receiving power from your AC adapter, you should look at the small display on the computer (not the green light on the adapter). If the computer is receiving power from the AC adapter, there will be an icon of a power plug on this display.