Gronk, that same thing happened to me when I tried the Zork edition on my 350. At first it was ok, then I poked around settings and changed the base font from the standard Sony and that put the reader in a nonstop reboot loop. Like you, I've been using PRS+ for close to 2 years without any problems and wanted to check out Zork.
The fix was easy enough though, I downloaded the uninstaller that's linked on the Zork site, made sure the reader booted up ok afterwards and nothing was left on it and everything worked ok. Then just reinstalled PRS+ and I was back in business. I did not try Zork again, since I already had done extensive testing with fonts and sizes before, I have one I'm so comfortable with that there really was no big advantage for me to use Zork.