My first 3790 last year had a problem from day one that it could not use speaker in the cradle and also could not get traffic update. I had to send it back to retailer as it was within first 30 days of purchase and got another one that worked perfectly. At first I have contacted Garmin and they have sent me another cradle free of charge and there was same problem, so I ended up 2 perfectly good cradles. This one has been good, so far... Saying that, in UK we have fair use policy of 5 years and if the unit fails within 5 years of purchase, you can get refund for the remainder of 5 years to use with the same retailer.
If your unit is still under warranty, contact Garmin for repair....more likely than not, you'll get a new unit......