So for the past 5-6 months, my pc would randomly freeze for maybe 15-20 seconds, then resume to normal.
This past week, it started randomly freezing for 60-90 seconds, and then it would restart itself. Any help on what it could be?

Start with formating it if it persists then we can check hardware.
Theres so much more info we'd need to address the exact issue but this way would be the fastest.