Sounds similar, I have the scope zeroed, Sometimes it stack several shots, and I think, "I love this Gun", and then it just starts throwing fliers, mostly low. I was shooting from my sand filled, leather shooter bag. I practice the artillery hold, I keep my hand off of the front of the rifle.
Like HNT5 said, bet it is lock up issue.Brass washers from www.airguntoys.com fixed my lockup issue, also had the pivot bolt problem with it not staying tight, drilled and tapped a set screw for that, replaced trigger 10 minutes after taking the gun out of the package. I have also replaced the piston seal, which got raid of the first 10 shots in a vertical line at 30 yards, added more power, and surprisingly less kick.