http://nikonf3.com/NikonF3User.pdf

You'll notice the MLU lever is oriented around the DOF button. Possibly related, but right below that is the mechanical release lever (which sure sounds pretty suspect here). Are you hitting the mechanical release lever and *not* the DOF preview? The DOF preview is just a silver button you press in to stop down the iris blades mechanically.

Have you tried actuating the backup shutter while at the same time exercising the DOF preview button while the manual shutter is still depressed?