While there can certainly be something going on with the OP's rifle, when I experience a similar situation, I generally try to eliminate shooting error on my part......and more often then not, this ends up being the case. While it may not be conscious to the shooter, concentration plays a huge role when shooting sub.5MOA and is perishable with repetition. Instinctive archers are very much aware of this. Shooting a rifle is no different. Sight picture, trigger squeeze, hold, position, etc. have to be exactly replicated to shoot with sub .5MOA consistently. Also, frequently overlooked, a light wind even at 100 yards can easily open a group from .25 to .5MOA. At 100 Yards, parallax has to be set perfectly, and may need checking throughout a shot string. If I see a grouping issue develop while shooting 100 yards at the bench, I"ll take a short break and switch and shoot prone at 200 yards to re-set my brain....and the rifle usually proves out fine. IMO, shooting from a bench can produce long strings of very small groups particularly when free-recoil is employed. For me, a bench makes shooting tight groups much more difficult with my "hard hold" hunting rifles due to the angular position of my body. For a hard hold with my hunters, my best groups with long strings have been achieved from a proper prone position, preferring 200 yard testing and zero setting over 100.