I did have this issue for a very long time and I ended up fixing it by changing the PSU, but this doesn't seem to be the issue in your case.
There's not a lot than can go wrong so it's either the CPU or the motherboard.
It became really bad at one point, crashing everytime,..
I may of fixed the issue, it's been running fine for a few days.
• removed any extensions cables to the GPU
• dusted off the PC
•updated the bios