Thanks @cankerman - the reason I asked is what you describe would indicate to me it is possibly a bit of manufacturing debris interfering with the movement of the striker, but not all the time. No guarantee of that...and obviously even if that is the case they should have found that at Ruger...but if you haven't already sent it back again you might want to do a detail strip (if you're comfortable doing so) and use some canned air and solvent to flush and blow out the striker channel.
I know my LC9s was kind of "messy" with manufacturing debris/gook when I got it. I think the Ruger is the only gun I've bought that really needed a good cleaning out of the box before the first range session (aside from the old surplus SKS I bought many years ago that was still packed in cosmoline). I always clean/inspect any new gun from ingrained habit but the Ruger was the first that I felt like it absolutely needed that bit of attention.
It sounds like they may have just followed a standard "customer reports light strikes" protocol of replacing spring and striker, for example, and then fired a couple of test shots and it went bang so they said, "fixed." I'm not prescient, so that's just a guess, but a fairly reasonable one and worth checking yourself before going another couple of weeks without the gun (unless you aren't comfortable doing a detail strip or, of course, if you've already sent it back again).
Of course, it could be a more serious issue like a sear engagement that is not quite right and sometimes allows the striker to release before being fully "cocked" so if you don't find something that is an obvious and clear fix send it back to Ruger for sure.
John