@nubreader: Not really ^^. I'd guess some kind of weird framework+fuse issue, where each one is trying its best to keep hold of the file... One another weird idea would be to try to switch fonts only when the Kindle is asleep, but that's pretty much a shot in the dark ^^
@gargoyle67: Er, the font package is just that, fonts. It won't do anything except put 12 files in a directory without the fonts hack

. So, really, it doesn't need an uninstall, it just puts files in the linkfonts/fonts directory

.
And an U007 error usually means that your Kindle is not jailbroken

.
EDIT: Just tried (again), the Fontin package, and it works fine here. It did convince me to switch Sans & Sans_Bold to the unhinted ones, because the hinting on these ones is horrible

.