I'm wondering if it's related to the hissing issue that Inu found (and fixed). Seems like some extreme oscillation, that's cured once the amp gets a little more to a stable operating point. Then, once the pot is replaced, the other pot in that battery is already at a more sane location and the issue goes away. This would sorta make sense, given the fix that Inu performed (see link I posted a while back).