And, in the Impact the .177 is the most difficult of the impacts to get great accuracy out of...
Sand the magazine, but first, do try single loading, to eliminate any possibility of that creating problems. You can make a single shot tray quickly and cheaply by going to Staples, getting a big pencil eraser. I used a drill, but a sharp exacto knofe will allow you to make a tray to feed the gun. There are pictures on line for the shape you need, and 20 minutes should get you something you can use.
I have 3 barrels for my Impact, and the .177 barrel is the only one that frustrates me. I shoot Field Target and it isn't as consistent as my Pulsar. It also requires discipline in shooting well- the form factor isn't conventional, so from the sticks, proper repeatible, hold and grip are required, not to mention scope alignment.
The .177 has the smoothest shot cycle, is the quietest (backyard shooting, anyone), least expensive to feed, and perversely, is the most rewarding when the shooter does everything right.
That includes lots of chrony work, checking every screw that moves when cocking/ firing, every session ( and there are a lot of them) and throwing away the first shot after refilling. My FX likes running @ 825 fps for accuracy with JSB 10.3's . The reg, using the crap stock gauge, is near 100 psi.
The higher I go beyond that fps, the more fliers I get. Do try a slower velocity (there is a too slow speed- FYI) and let us know?
You might want a turnkey gun to shoot while you fiddle with the Impact, so the frustration isn't as..frustrating? Seriously consider getting something like a Beeman P-17 that you just shoot, enjoy for what it is, and then go back to the bench to work on the FX. It may be a bit of a ship in the bottle at first, but you'll learn tons working on it!