I don't disagree with the other posters at all. The issue may be as they've described. However, it could be other things too, and certainly hardware related.
Could be ram, thermal throttling (on either the CPU/GPU), cooler not working (fans not spinning) etc. Lots of reasons. Test your ram, with a bootable USB with memtest on it. Lets rule that out straight away. Check your fans are all spinning and internally the PC is dust free.
I think both are on the right track, and I would advise the same - monitor whats going on. Use something like Hwinfo/HwMon as it's more comprehensive, and has all the info you need in one screen, that includes, CPU/GPU temps, mobo temps, voltages, CPU/GPU usage levels amongst a whole array of other metrics. Install one of those and run it, take a screen shot and post it here. It will help rule out some stuff, and get an accurate picture of whats going on with your system. Also, if you can post a list of your exact PC specs, make/model etc, including PSU model that would help a lot too.