I'd actually be quite interested in what anyone else here has to say about this as I've had this exact same problem for several months now, perhaps a year. Likewise it seems to be a sort of "warm-up" time as when I've been using it recently it doesn't have the same problem or a reduced duration at least. I would assume it's a problem in the amp section, but the specific issue and any potential solutions (or the cause) still elude me.