MacBook Pro (mid-2012) SSD Problems
I used to work at an Apple certified repair center and would definitely suspect the SSD. Things you could check:


Reseat the sata connector on the logic board side
Try a different sata cable
Try a different ssd
Put the ssd in a diffent machine and see what happens


Unfortunately not everyone has multiple ssd, parts and machines to test with. Can you try and claim warranty on the ssd?