You should post your own example file(zipped) in this case, since I think there is more to it then the steps you provided. In the mean time, try pressing the Reset Plane button, see if that helps. As for the steps you provided, I get a 45 degree cut every time. The only way I can make it not work is if I rotate the object in sub-object mode, then it will still be a 45 degree cut in world space, but not object space. You could adjust the pivot so it's got the same rotation as your object, then use the slice plane and reset plane, see if that works.
MP