If it was springing open, it had a lockup issue--how was it resolved? Did the rifle go back to the factory for repair, was work done on the latch or underlug? Did it just stop, for some unknown reason?
Without being there to observe your technique, it's hard to say, but could it be that you are holding the rifle a bit differently each shot? or that one time you're leaning into the rifle, and the next time, not or not as much?
Do all the high hits group, as well as the "zero" hits, are you getting two distinct groups, one at POA and one 1.5" higher?
How about your ammo- is it factory or handloads? BTW, what caliber are we talking about, anyway?
How many rounds did you fire before getting disgusted/frozen enough to leave?
What distance were you shooting at?
Temperature can have a large effect on how ammo works--for instance, a round that sits in a warm or hot chamber for several seconds will shoot different from one that's been out in the cold and gets chambered and fired in a shorter interval.
Whatever is going on, if it's giving two small groups 1.5" apart, at, say 100 yds, then I wouldn't be too unhappy--but it would be nice to know what's going on.