iMac Mid 2007 iMac Running Very Slow After Fresh Install of Yosemite

I have a 24" iMac Mid 2007 that was running fine up until a few months ago.

Suddenly, the Mac would not start up and get stuck in the Apple logo

bar and I ended up re-installing OSX 10.10 because repair disk permissions

would give an error that forced me to wipe the drive and install everything again.

(My data is backed up luckily)

Eventually I was able to wipe the disk, after many tries, and the computer

started running very slow.

I assumed it was a bad disk or bad ram. Looking at Activity monitor though,

no CPU load was detected, high RAM usage, nor abnormal disk activity R/W.

I downloaded Remember (a GUI for memtest) and ran several loops and

found no errors in RAM. It might be the HD but when I run the Apple

Hardware test, I get a grey folder, and doesn't start up. The HD is relatively

new, replaced in 2013, so I would doubt it is that but it could be.

Lastly, I tried installing El Capitan in hopes a fresh update would help everything...

And no, after many failed attempts at the restart button of OSX install App, the

installation would fail... After 3 or 4 tries, I was able to complete the installation all

the way until restart and when the grey Apple logo that says "Installing software"

stopped at 28 minutes going back and forth between that and 30 and it never installed.