My first question is what type of ammo are you using in 2 3/4? the next is what is your gas system set on? i have had success using Winchester AA 2 /34 shells on #2 setting with very few stove pipes. Also congrats on your new S-12 hope we can help you out with this issue, so you can get to the enjoyment.
I found an IZ-107 S-12 24"bbl for sale on gilbertsguns.com for $439.95. I bought my S-12 from them 2 weeks ago, same model with the factory BHO, for $460.00 shipped. I am not affiliated with Gilberts in any way,i just bought my first Saiga from them, just passing on a find to anyone whos looking for a S-12. Good hunting and good luck, Justin