We have a Weatherby Vanguard in 300 Weatherby Mag with a VXII 3x9 50mm scope. The rifle and scope are about 20 years old. When first purchased, we had a problem with scope slipping in the the scope rings. Added a couple of shims to the rings and the rifle/scope has held zero and operated flawlessly since. It has been used the last few seasons with no problems. It has not been dropped or jarred but last season the variable adjustment became very stiff during cold weather. It is inside now and continues to be very stiff. Checked the gap between the scope rings and the variable adjustment ring and it is fine with no slippage in the scope. Before I send it back to Leupold, any ideas or suggestions?