Can you ask someone else to shoot it and see if they get the same scenario? That would quickly rule out the user error factor.
Then, of course, repeat with other ammo. Rule out the "gun doesn't like this ammo" factor.
How's the condition of the crown? The bbl and rifling in general? As long as you're taking time between rounds, how about run a patch down the bbl between rounds. Maybe the rifling is (sometimes) peeling off a piece of jacket which is then getting pushed out by a subsequent round.
All just wild guesses here. Only time I've had anything similar is with sudden and strong winds with long range 22 that had time to slow considerably by the time it got near the target.
Sgt Lumpy - n0eq