Regarding the bobbin case just slipping around...
If you take off the plate that goes around the feed dogs (I think it is called the sewing plate), be sure that when you replace the plate back on, that the bobbin assembly is in the right place. If it isn't, when you put the bobbin case in, it will just slip around and won't catch in the right place. However, if you are already sewing with it on fabric, then you must have gotten it in the right position. Ask me how I know. It can be very frustrating if you don't know that little tidbit of information.