Have an accomplished shooter fire the weapon, if their results mirror yours, that points towards the gun.
Second the identifiable group comment, hitting everywhere in that corner is not enough to go on. Bring it in closer.
As a lefty, I will venture to stab in the dark that your right hand is giving a sympathetic squeeze when you break the shot with your left index finger.
Concentrate on dryfire at home and see if the problem goes away. Try dime drills, placing a dime on the front sight and dryfiring without it falling off.