Unfortunately I have some experience with this, but also a solution.

The Leopold VariX-III 3.5-10 AO on my 300 Jarrett was mounted in the Leopold one piece base Kenny mounted in 1992. It had over 1200 rounds through it at the time. The first two animals were shot at 35 yards or less in bush, so I don't know when the problem cropped up. An impala at around 150 yards was facing left, and gut shot. The follow up shot nearly 30 min later was made at around 60 yards, and was well behind the shoulder. This was also well right of point of aim. Immediate return to camp confirmed it was hitting just over 8" right at 100 yards. Elevation was fine. I re zeroed, and finished the safari with a "sighting shot" each AM. It went to Premier Reticle when I returned and they confirmed the "erector" was bad. Replacement was free (they're Leupold warranty approved), and it's been fine since.

The 300 Jarrett's not a 505 Gibbs, but it is a very "sharp" recoiling rifle, and abusive to scopes. Premier Recticle said the Erectors in the VariX-III scopes is very good, but should be replaced occasionally. It shouldn't be looked at as a flaw, just maintenance. The solution, send it to them a few months befor eany safari for a rebuild, and pack a spare. If you have Premier Reticle replace all your reticles with the EXACT same one (German 4A in my case), you'll never notice the variation when you use your spare or swap to a different rifle. kind of nice when you practice with a 270 but hunt with your 458 Lott.

Safe Hunting!
Clint