The S400PX cells sold are generally a 395 silver cell with the collar. On their own the 395's are about 1/6 of the cost so it is worth swapping the ring over. Because of the difference in chemistry the polarity on silver cells appears different. On a silver cell the case is + and the stud -. On mercury cells it is the other way round. With the Spots, I find the 395's work perfectly well without the collar.
You may want the check the witing under the base plate. If this is corroded, it could give you the sutoms you describe.