Try downgrade with old 2.80 firmware and then Clear non vol.
Then use prebusb to format your nuvi and after format, create new folder and add name Garmin
Turn nuvi off/on and then update with webupdater:

Code:
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Update only FW, don't update voice or other files.
When Webupdater write that you have installed 3.60 press buton cancle in webupdater and turn nuvi OFF and On again.
Now your nuvi will update and you will see if you ar still in reboot loop or nuvi turn on normaly.

If turn on normaly, then run again webupdater and download voice and other files.