A few weeks ago i had the same probleem while geocaching. My Oregon 700 was working fine.I was trying to find a chirp cache, so i turned the chirp function on in my settings. After about 20-30 sec my Oregon stopped working and each time after i turned on my gps it shut down after aprox.20 sec.
Turning the chirp function Off again solved my problem.
A friend cacher had the same problem and it was solved by turning off the chirp function.