Then it's likely either overheating, and you can monitor that by downloading HWinfo or CoreTemp and watching the CPU and other thermal values up until it shuts down to see if it's getting hot, or you've got a power supply problem. Most commonly, this would usually be a power supply issue.
It's also vaguely possible that you might have a failing motherboard capacitor that is taking a while to heat up to the point where the protection kicks the system off, but I'd lean far towards power supply if you don't find anything off with the CPU core temperatures.
What are your full system specs including model numbers for CPU, motherboard, graphics card, memory and most importantly, power supply?
When it comes to temperature issues, especially if this is a build that has been running for a year or more, taking care of the basics first might save everybody involved a lot of time and frustration.
Check the CPU fan heatsink for dust accumulation and blow or clean out as necessary. Avoid using a vacuum if possible as vacuums are known to create static electricity that can, in some cases, zap small components.
Other areas that may benefit from a cleaning include fans, power supply internals, storage and optical drives, the motherboard surfaces and RAM. Keeping the inside of your rig clean is a high priority and should be done on a regular basis using 90 psi or lower compressed air from a compressor or compressed canned air.
Use common sense based on what PSU your compressor is set to. Don't "blast" your motherboard or hardware to pieces. Start from an adequate distance until you can judge what is enough to just get the job done. When using canned air use only short blasts moving from place to place frequently to avoid "frosting" components.
If this system has never been cleaned out, it could be very possible that there is dust packed into the power supply causing it to overheat, or in the CPU cooler heatsink fins. Either one could cause your issue if it's not simply due to an old failing power supply.