I just bought a new 912, and had seen on variouse posts that some people were having problems with 2 3/4 in shells, and that the smallest reccomended load for cycling is 1 1/8 ounch. Quite a few people mentioned shooting a few boxes of heavier magnum shells through followed by a good cleaning to solve the problem.
I picked up 3 boxes of 3 1/2 mags to break in the gun but decieded to try some 1 1/8 oz trap loads first, and only had one feed problem on the 6th shot. Since it was cycling so well, i decieded to try some light trap loads with 1 oz shot, and again it cycled fine.
I would definatly check the piston, to make sure it is set to light loads. Just line up the stamp with the rear of the arrow, with the front stamp of the arrow with the side marked light. (basically the two halves should look like a continous arrow.
Before i ever shot the gun i did tear it fully appart following the owners manual instructions and gave it a good cleaning and lubercation (used browning shot lube (comes in a aresol can that looks like a shotgun shell). followed by a good wipedown to wipe off the excess. Upon doing this i did find quite a bit of thick packing grease which i removed especially on the bolt carrier and in the trigger mechanism.
I would say tear the gun down and give it a good cleaning and lubercation making sure you get all the packing grease out, and confirm the piston is set to the right position.
Also there is a o-ring which goes over the magazine which stops the piston from sliding all the way off. You might want to make sure this o-ring missing. Its easy to loose if your not paying attention. I would assume the o-ring allows for a tighter forward seal, and prevent gas leaking too quickly out the forward end of the piston collar, which may play a roll in cylcing the lighter shells, but may not affect the higher pressures generated with 3 and 3 1/2 magnum shells.