After a couple of similar panic issues with my now old nuvi 250 I managed to fix it with the "Krok 1 CURE" after weeks of searching for a cure. I had even replaced it.
My advice for ANY nuvi or hand held - send your unknown hacked mapsets to a (micro)SD card instead of the internal flash RAM! Cards are inexpensive and YOU CAN REMOVE IT if you have any problems.
I learnt the hard way on more than one GPS.. :hammer: