I have a Stevens High Power 425 in 35 Remington caliber. It does not feed the cartridge properly. Favorite gunsmith is stumped.

Here's what is happening. As lever is worked,the carrier flutters letting the cartridge half way out of the magazine then stopping. When I bump the lever the carrier then drops back
down letting the cartridge proceed back and be levered into the chamber. We installed a new magazine spring so there is adequate spring pressure to feed the cartridge back.

Please help me. I do not have a history of this rifle but want it to work and go back to shooting whitetails.