I had a similar experience to Alleyyooper, though I didn't attribute it to too much spin. My accuracy would degrade when I shot shells with a high muzzle velocity (i.e., SST's). When I switched to something slower (Copper Solids), accuracy was fine. My theory was the rifling had trouble biting into the faster rounds, which would then slide across without picking up as much spin, and I'd end up shooting knuckle balls. (which was probably compounded by the fact that my slug gun had a short barrel). 3" shells probably are generally faster than 2 3/4.

I switched from a 12 to a 20 gauge a few years ago, and am glad I did. I actually enjoy shooting my slug gun now.