My computer auto restarts

i5-7600k @ 3.80GHz, 16GB ram.

Regular use pc. No OC.

Recently when I launch game applications, my PC auto-restarts after 15-30 minutes. Updated graphics driver but it persisted. Found out my CPU was hitting 100% so I guess that was causing the restarts. Now, opening new tabs on Chrome or clicking links also causes it to spike to 90% for about 5 seconds. Wondering what is causing my CPU to hit 100% usage. It's not overheating either. Thoughts are 1. HD issue, 2. chip issue. Which is more likely? And what else could it be? Thanks for any info.