Some vertical blade play may develop if the stop pin is somehow worn or crushed (repeated inertia openings???), or if the tang has worn to the point of the lock travelling accross to the opposite side, pretty much like any variation on the liner lock.
Try rotating the stop pin to another position. Other than that, I don't think there's much else to be done. Either way, that kind of play probably has no impact on safety or performance.
I had that same problem on a Para I got used, and have no issues with it after tweaking for minimizing the play. For the record, I have four Paras.