Lacking some information necessary to determine the cause of your problem I will point out some possibilities to look at: if the stock to action screws on a Omega are over tightened it will cause problems especially on the plastic stock. If the touch hole in the breach plug is flame cut out especially if it is irregular can cause your problem. The best solution for that is to replace by drilling tapping and putting in a vent liner.
And yes even good stable old Pyrodex can sour if exposed to moisture long enough though it is not near as bad as the APP powders, the only powder I would trust to set any length of time is Blackhorn. The best answer to that is open a brand new can and try it. There is also the possibility that the scope has been dropped or banged some stand up to this better than others but if a gun is dropped or knocked over and it bends the barrel of the scope even a little the parallax induced will cause the results you are experiencing.