Sounds like overheating to me. Get GPU-z to have a look at the GPU temps and CoreTemp to check the CPU. If you're seeing high temperatures, that's most likely the cause of your troubles. If you built the PC yourself, you should be confident enough to replace the thermal paste between the CPU and the cooler. It may be a little tricky with certain cooler/motherboard combinations, so look up a guide on YouTube. GPU paste replacement is more complicated and often doesn't work, but performed with care, is certainly doable by a novice armed with YouTube. Anyway, check the temps and let us know. And post your PC's specs.