Duey,
First, I won't use anything but Kadee couplers, except the "exact scale".
That stated, are you using the "truck mount" coupler boxes that Athearn passenger cars come with ?? The "arms" can warp,or sag, and cause coupler mismatch. You may want to think about bodymounting the couplers.
Do the cars uncouple consistently at the same place on the layout?? The longer the car, the more vertical movement of the coupler face due to humps or dips in the track. You'll find this with auto racks, 89' hicube box cars, and 85' flats.
Magnetic "hoses" uncoupling pins; When I had them (I'm clipping all of them as I use the rolling stock. When you have to remove 40' of train from the layout quickly, as I do on the modular layout, at shows, the pins get in the way.), I always adjusted them higher than the gauge just to insure they didn't snag.
Coupler height adjustment is a must! The tighter control you maintain on coupler height, the less chance of uncoupling.
Kadee clones. On the modules I run long trains, and the drawbar pull will cause the plastic shank couplers to work vertically, and uncouple even with the height set correctly, and the track as level as possible. I will only use metal couplers, with coil springs on the knuckles.
I have "operated" on several club layouts, with my own rolling stock, and have never had any problems with uncoupling, or derailing. The effort pays off.
Pete