Constant Restarts

That means you likely did not re-format the HD when you initially bought it. Most EHD's come pre-formatted to use with MS Windows. You should have erased and formatted the External Hard Disk to work with your Mac.

My guess is it may be an issue with one of the many external drives or other peripherals connected to the iMac that is causing your issue. I would start by shutting down the computer and physically unplugging all wires peripherals (External Hard Disks, printers, cameras, scanners, etc... and any hubs that plugged into the Mac) and the restarting and testing for a couple of days. Then slowly introduced each peripheral 1 by 1 giving each at least 24 hours before introducing the next until your machine crashes. Next remove the last peripheral introduced again and test once more again, if the machine then is stable then you have found your culprit.

I also noticed you are using Paragon NTFS, I am assuming this is to share an External Hard Disk with a MS Windows machine, are you aware the Paragon NTFS is a hack?

Great, well next time you buy a drive please be careful to check what it has been formatted as before putting it into use. If there is any software installed on it, please consider that junkware that should be tossed!