Are you able to reproduce your problem predictably? Also, how often does it happen? If it doesn't happen all that often, it's entirely believable that the drive may pass Dell's tests, and hence they do nothing.
If you happen to have another Mini available for testing (or "checking out" a friend's/coworker's Mini), I would suggest swapping the SSD
to see if the issue goes away. However, whether you are even willing to do that or not, I think you have to deal very firmly with the Dell tech support to have them send you a new SSD
. I agree with you that it's highly likely there's something wrong with the SSD
, though the mainboard could possibly be the culprit.
Just keep at it with Dell. Don't go away until you get the problem fixed. If they won't help you, hang up and call/chat again.