How are you transporting and storing it?

Maybe the barrels orings are worn and not keeping your barrel true

I am using one of these now, as I cant believe how floppy the barrel had become, but with this on, the barrel is as solid as my Steyr .243. I can see why they modified the front barrel support on the mark 3 with a grub screw!!

I was planning to drill/tap and fit my own grub screw as I now have a Bridgeport Mill - but the barrel support is working perfect for now.

Infact the barrel band is so stiff that I dont get POI shifts when I switch Silencers! - well not enough shift to worry about!

https://www.ebay.co.uk/itm/255035000630

To add what FXLover asked.... What pressure is your tank at before you store your Impact versus the next time you grab it to plink off a rabbit etc? Older Impacts (First Gen) had issues with regulator creep but FX soon fixed that issue with updated versions as their newer generations came out. My old first gen had a regulator creeping issue till I replaced it with a Huma. And when you chrono are you starting at a full fill? These rifles are setting picky...sort of...which I'm sure your aware of...and need to settle in after even a small adjustment, usually a full magazine or two. I'd suggest checking your tank pressures first, if you haven't already, for a potential slight leak=pressure loss and could explain that much of a foot pound variation. Also check allllll of your set/grub screws, sometimes they come loose over time.