I have just written to Garmin support regarding the long and winding route that the 3790T takes compared to my old and trusty 760. Other than that, I only have the 3790T for a few days and so far, working fine.
Do you have the latest map updates? I have found that some of the odd routes you get sometimes tend to disappear with newer map updates. But I am still waiting for my 660 to tell me to just go make a left turn to get home, instead of the three turns onto a narrow road it insists is a superior route!