It could be the power supply, but it can also be any number of hardware components...
Take the box apart, blow it out and reseat all of the boards, chips and the processor.
Does this happen when the machine gets too hot? When the processor is in high use? If so, try running it with the case apart and a fan blowing COOL air on there... If that works, you might just need to increase airflow, but neatening up the inside, or adding more fans.
Caution though, you can damage this by working on it, so your warranty could be void... But this is exactly what they'll try to get you to do, anyway.
Next, also try to run the system with a minimal configuration - by this i mean detact everything that you don't need to boot up. Run the machine a while and see if you still have your issues. If it runs well, shut it down and add one more item at a time until you experience the problem again. Once you think you've found the cuplrit, take it out and continue with everything else again as there could be more than one defective part. [especially if power was involved with the problem, one piece can fry others]
Good luck