Dunno if it applies in this case, but FWIW, I've been getting very aggravated this late summer/early fall time frame, because I switched from a fixed 10X scope to a higher power variable. Went from little one hole groups (sub half-inch) @100yds to 1-1.5"groups w/ a Sightron 6-24x!!! I was *not* a happy camper!! Now, as it turns out, the Sightron had some other 'issues', and is back at the factory. But I have pretty much the same problem w/ the Weaver V16 that's on the gun right now. When I crank it to 16x, I get bigger groups than when I am at 10-12x. Someone at the range finally pointed it out: That range is kind of a poor design, and has very, very bad mirage, and it's in a canyon which causes some funky switching winds. At 16x, the mirage screws up my shooting. Down at 10x or 12x, I'm usually OK, unless I'm shooting a long string, at which point barrel heat causes enough mirage to force me to lower magnification to 8x or even 6x.
Like I said, I don't know if this is the case w/ your gun, but it wouldn't hurt keeping in mind.
Monte