Bought 3 firearms the same day each one from a different dealer without an issue (Well not from NICS just Chase Bank). 12 firearms and I've never had a purchase put on hold.
It doesn't seem random enough to be completely random though. Some folks have never had a delay (LawDog and myself) and some get delayed on a regular basis. There must be some flags that go up like a very common name shared by a bajillion people, or something. Maybe every third Tuesday and fifth Friday they delay everyone?