oatmeal or oat bran, stone ground flour, etc.
We need an answer from a real baker here, but I can give you my observations. I've done a fair bit of baking, and found that I had crumbly interiors if I let the dough rise too long (sometimes it seemed OK, but the baked bread had too much air, and was dry in the middle), or if I used extras (oatmeal, stone ground flour, etc.) that lowered the gluten content of the dough, leaving less with which to build the framework of denser bread. It's just not sticky enough. You could try adding 1/4 cup wheat gluten to your flour mixture, and see if that solves your problem. The only other thing I can think of is that the baking temperature may not be high enough, causing it to take longer to brown the outside, while giving ample time to dry out the inside.
i'm no expert but i think i've read that improper kneading can cause the crumb to be less than desired. the stretching of the dough is important for the crumb to develop. let's see what others say about that. but maybe you need to knead longer than you currently are. likewise if the rise time is too long after a proper kneading, the stretched structure could collapse and ruin the texture. that's what sarah k speculated also. does this happen with one particular recipe or with all the ones you try? have you tested your oven temperature with an oven thermometer?
It's usually the grain ones such as oatmeal or multigrain. I use the window pane test so I think the kneading is on target. I think it might be that I am using my Kitchenaid too much and possibily incorporating too much flour. Could this be the answer? I think the rising time is appropriate and the oven temp is ok too. Could too much flour make it crumbly in center? The end pieces are ok, but when you get to the center the slices crumble unless we cut thick slices!!!
If there are large air bubbles, it could have risen too long. But also make sure that there is enough gluten; using bread flour or adding some vital gluten (available at Whole Foods and other health food store) will increase the "stretch" of the crumb.
I'm not an expert either (it's been over 10 years since I made bread with gluten in it), but I do know that if you break the dough as you knead it, you'll break the gluten strains and cause an odd crumbly texture. You want to knead your bread in smooth fold-over motions that never break those strains. The Kitchenaid dough hook should work fine for this, but be careful how you move it around the hook and take it out. Also, too much flour could be contributing to the crumble.