Q: El Capitain & Sierra keep freezing after fresh OS reinstall.

I am using a 21-inch iMac (late 2012 - purchased brand new in 2013) which has been functioning well until a few months ago, when it started freezing gradually (the infamous spinning beach ball) until a few weeks ago when it became unbearable (It was running Sierra back then).

I wiped out the computer and reinstalled Sierra but the problem persisted, then I wiped it out again and installed El Capitan and the problem is still here.

I believe I am a gentle user (i.e. always quit/delete unused apps and media, rarely use 3rd party apps, besides disabling all unnecessary features from genie effect window-minimizing, to animating apps at launch etc.).

I have 8GB of RAM and 1 TB HDD of which I am using no more than 20%.

Activity Monitor (Memory) shows I never use more than 25%.

Activity Monitor (CPU) shows spikes every time the freezes occur - which is extremely random and can happen when no heavy-lifting is taking place. Rest of the time CPU load is below 5%.

Apple Hardware test reported no problems.

Disk Utility First Aid in Recovery Mode reported no problems.

Safe mode runs fine.

There are no Genius Bars in my country

I am not upgrading to SSD (yet) because I didn't spend $1.5K on a computer to have it upgraded in less than 3 years! Even my old Microsoft computers lasted longer than this.

Everything is backed up.

Urgent help needed!

Thank you.