When you boot into Rockbox, on the H300 series no part of the original firmware ever gets run, so problems like you experience (extremely short runtimes in Rockbox) shouldn't be able to happen because of the original firmware (to my limited knowledge at least, which I'll admit in this area is fairly minimal: I'm making an assumption here and that's never safe in the long run, the assumption being that the original firmware can't leave something that gets in the way after a power cycle)