crash during heavy computing/ Hard drive?

Here are my specs:

MacBook Pro (Retina, 13-inch, Early 2015)

3.1 GHz Intel Core i7

16 GB 1867 MHz DDR3

Intel Iris Graphics 6100 1536 MB

128GB OEM SSD

Running 10.12.6 (Sierra)

I bought this computer used on ebay after my 2011 Macbook Pro crapped out on me. Initially the computer ran fine. Then after installing High Sierra, I experienced intermittent crashes that I believed to be kernel panic issues, so I created a bootable version of Sierra and downgraded the computer back to it. While it is now much more reliable, I still have issues when doing anything heavier than running word processors or surfing the web. The two latest crashes I have had were while downloading -don't laugh at me- fortnite which crashes everytime I try to run the downloader. I started to suspect that it could be a Hard Drive issue so I downloaded Blackmagic Disk Speed Test and have experienced crashes from running that application.

Any light anyone could shed on this would be greatly appreciated.