Hi...what would cause a Hamilton 950b (double roller) to run for only 8 hours and then have to be rewound because it had "rundown"? It seems to rewind fully. Prior to this it was always good for 24 hours or more on each rewind. Rewind stem does NOT appear to "slip" when winding. Can a Mainspring "break" and still funtion partially? How does that happen? Or is there somewhere else I should be looking?Thanks. Robert Chico, Calif.