Get a can of compressed air and a small brush.
Give the innards a good cleaning.
Pay attention to the heatsinks and fans.
Don't forget to give the psu a few shots of air from both directions.
Leave the side panel off and test.
CPU: AMD FX-8320
GPU: NVIDIA GeForce GTX 960
MOTHERBOARD: ASRock 970DE3/U3S3
RAM: G Skill F3-12800CL9-4GBXL (x2)
POWER SUPPLY: Antec HCG-620M (620W)
STORAGE: Seagate ST1000DM000-9TS15E
OS: Widows 10
My computer isn't that old, but recently it's been shutting off seemingly everything except the fans while running games. I've noticed that it only shuts off while running graphically intensive games, so I tried turning the gpu fan up to 100% in open hardware monitor, but it didn't help. I'm thinking it could be the power supply? Don't those fail after a certain time?