Hi Pastor Dan,
First, welcome to Glocktalk. You'll get some good answers to your questions here (mostly below my post).
On your issue, I would think it would be worth it to try some Federal buckshot that uses a "Flite control" wad. I've heard good things about it keeping a better group for people. Hopefully smarter people than I can come along and verify this.
What choke does your slug barrel have? I believe my similar barrel has an improved cylinder choke. It works just fine (for me) with plain old Federal 4Buck. YMMV.
That said, how bad does yours pattern? At what distance? What are you using currently for a load? What choke is in your gun. These types of questions might help people better diagnose what you have going on.
Edit: Here's the 00Buck from them that would probably work. They also have the same load in 4Buck.