Hi and welcome to Geeks to Go. Random events like you described are ususally caused by heat, memory, driver issues or the power supply. You will need to investigate each possible cause to either rule it out or correct it. Check the fans, make sure all are running. Also check inside the case to make sure it is clean. You can get "canned air" and blow out the dust. Use a vacuum cleaner in conjunction with that for thorough cleaning. You can test the memory with Memtest86 from http://www.memtest.org/
Make sure you have the current driver for your video card. If you update the driver, use Driver Cleaner to clean the previous installation first. (http://www.drivercleaner.net/
it is free). As for the power supply the real question here is, does the power supply have adequate wattage and current capability for your system, especially during graphically intense gaming activity. This type of activity puts a draw on the power supply and also generates heat. Look at the lable on the side of the power supply. It will give you the manufacturer, model number, wattage, and amp ratings on various rails (+12v, +5v, etc). Make note of these. Then go to http://www.hardforum...isplay.php?f=93
and check out the sticky posts at the top of the forum. This is the best collection of information on power supplies that I have found. You will be able to check your power supply against the information in the sticky posts and determine how it measures up. There are power supply calculators where you can detail your systems components and come up with watts needed to power your system as configured. Is it adequate? See that sticky post where a multitude of common power supplies are identified as less than desireable because the over state their ratings by they way they apply the term "under ideal circumstances". In these cases you need to deduct 30 percent from the stated rating to come up with a realistic rating. This was how I figured out my 420 Watt Raidmax power supply that came with my case was only capable of 294 watts (I needed 370 based on the calculator) and not adequate for my system. I had random reboot problems too. I replaced my power supply with a good quality Forton 520 watt PSU and reboot problem went away.