I've been having the same problem with my 660. It's just over a year old and I'm not a happy camper with it. I read that a diode will malfuction because of heat. This diode tells the unit if an external antenna is attached. When the unit can not find the sats, and no external antenna is attached, the unit goes tits up.
I bought an antenna off of ebay and will give that a try. If it works...great, if not, I'll get a TomTom.
Howard
March 2009